Evaluations of Quinone/Hydroquinone Couples Acting as Two Hydrogen Atoms Antioxidants, Radical Quenchers, and Hydrogen Atom Abstractors

Abstract

Quinone/hydroquinone couples play a crucial role in a variety of biochemical processes and chemical syntheses. Extending from our previous work, a practical dataset including the thermodynamic driving forces of 12 chemical processes for 118 quinone/hydroquinone couples accepting or releasing two hydrogen atoms in DMSO is established. The dataset serves as a foundation for assessing and discussing the thermodynamic capabilities of hydroquinones acting as two-hydrogen-atoms antioxidants or radical quenchers, quinones and semiquinone radicals acting as hydrogen atoms abstractors, and quinone/hydroquinone couples acting as dehydrogenation and hydrogenation reagents. The fundamental thermodynamic knowledge is expected to further promote the broader application of quinone/hydroquinone couples in the field of chemical antioxidation and redox reactions.

1. Introduction

Quinone/hydroquinone (Q/QH₂) couples play essential roles in the field of biochemistry [1,2,3,4,5], pharmacochemistry [6] and synthetic chemistry [7,8,9]. Within biological systems, quinone enzymes and their reduced states serve as hydrogen mediators to achieve the delivery of electrons, and hydrogen atoms or ions [1,2,3,4,5]. Propyl gallate, protocatechuic acid (PCA) and nordihydroguaiaretic acid, which feature a distinctive catechol skeleton, are renowned for their antioxidant properties and widely used as food additives to combat oxidation by donating hydrogen atoms to neutralize radicals [10,11,12]. In chemical reactions, Q/QH₂ couples operate as catalysts [13,14,15,16], particularly in electrochemical syntheses [17,18,19,20,21], where they generally act as hydrogen atoms or hydrides abstractors to initiate substrate activation or complex molecular transformations. Moreover, some well-known quinones [22,23], 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and tetracyano-p-benzoquinone, have been applied as dehydrogenation reagents to construct unsaturated bonds by oxidating amines, alcohols, alkanes and pre-aromatic compounds into corresponding imines, aldehydes or ketones, alkenes, and aromatic compounds. Additionally, a variety of synthetic methods are developed or designed to oxidize hydroquinones into corresponding quinones with potential physiological activities [6,24] and reduce quinones into related hydroquinones with potential antioxidant reactivities [10,25]. What is more, Q/QH₂ couples have already been proved to be a type of potential chemical hydrogen storage materials with the help of the electrochemical method [17,18,19,20,21,26].

It is found that the realization of redox functions for quinones and hydroquinones typically involves H₂ acceptance and release with Q/QH₂ couples playing the role of hydrogen mediators. Therefore, the thermodynamics on hydrogenation and dehydrogenation of quinone/hydroquinone couples as hydrogen mediators are critical physical parameters that provide a quantitative assessment of the thermodynamic capabilities of Q/QH₂ couples acting as redox catalysts or reagents, antioxidants, and potential hydrogen storage materials.

In our previous study, the bond dissociation energies (BDEs) of various hydroquinones (QH₂) and semiquinone radicals (Q_bH^•) in DMSO were calculated by DFT (density functional theory) methods [27], which lays groundwork and presents us a unique opportunity to thoroughly clarify the thermodynamic properties of Q/QH₂ couples. There are several reasons why DMSO was chosen as the solvent in this study. First, a considerable amount of thermodynamic data for hydroquinones has been reported in DMSO. These extensive experimental datasets allow us to verify the accuracy of our DFT calculations [27]. Second, the Gibbs homolytic dissociation energies of common antioxidants and Y–H bonds are typically determined in DMSO, which facilitates straightforward thermodynamic comparisons. Third, many antioxidant experiments have been evaluated in DMSO. The thermodynamic data provided for quinone/hydroquinone couples are thus useful for selecting appropriate antioxidants. Fourth, DMSO is an excellent reaction solvent. Numerous hydrogen-atom-abstraction-initiated reactions are conducted in DMSO. Therefore, the thermodynamic parameters calculated here can offer practical guidance for using quinone/hydroquinone couples as radical quenchers or hydrogen atom abstractors.

In this study, the thermodynamic properties of 118 hydroquinones are investigated. These hydroquinones encompass 69 p-hydroquinones (1H₂–69H₂, denoted as Q_pH₂) and o-hydroquinones (70H₂–118H₂, denoted as Q_oH₂), and the quinone forms of 118 hydroquinones are depicted in Scheme 1. Thermodynamic driving forces of 12 chemical processes for Q/QH₂ couples accepting or releasing two hydrogen atoms in DMSO are derived or calculated in accordance with Hess’s law [28]. The resulting thermodynamic data enable us to examine and clarify the thermodynamic properties on hydrogenation and dehydrogenation of quinone/hydroquinone couples as hydrogen mediators. It should be noted that the thermodynamic driving force is a key factor in determining the kinetic process and it can only give thermodynamic guidance and judgment.

2. Materials and Methods

2.1. Thermodynamic Parameters

According to the chemical transformations involving the hydrogenation and dehydrogenation of quinone/hydroquinone couples, a thermodynamic analysis platform [28] is established and illustrated in Scheme 2. For a chemical process, the corresponding Gibbs free energy stands out as an essential thermodynamic parameter to judge the spontaneity and quantify the equilibrium constant, as well as assess the stability of initial reactants [28]. In this work, the fourteen Gibbs free energies associated with the hydrogenation and dehydrogenation of Q/QH₂ couples are derived to provide a detailed quantification of the thermodynamic capabilities and chemical characteristics of Q, QH₂, and related intermediates, QH^•.

Scheme 2. Thermodynamic cycles constructed based on the chemical processes of hydrogenation and dehydrogenation for quinone/hydroquinone couples.

Scheme 2. Thermodynamic cycles constructed based on the chemical processes of hydrogenation and dehydrogenation for quinone/hydroquinone couples.

For the chemical reaction of asymmetric QH₂ releasing two hydrogen atoms or H₂, the reaction unfolds in a series of discrete steps (Steps 1–6), and the thermodynamic capabilities are quantified by the corresponding Gibbs free energy changes in Steps 1–6. Step 1 is the chemical process of QH₂ releasing a hydrogen atom to generate semiquinone radical Q_aH^• owning more substituents close to the resulting O-radical center, QH₂ → Q_aH^• + H^•, and the thermodynamic capability is described as the corresponding Gibbs free energy, ΔG_HR(QH₂). Step 2 is the chemical process of Q_aH^• releasing a hydrogen atom to generate Q, Q_aH^• → Q + H^•, and the thermodynamic capability is described as the related Gibbs free energy, ΔG_HR(Q_aH^•). Step 3 is the chemical process of QH₂ releasing a hydrogen atom to generate semiquinone radical Q_bH^• owning less substituents close to the resulting O-radical center, QH₂ → Q_bH^• + H^•, and the thermodynamic capability is measured by the corresponding Gibbs free energy, ΔG′_HR(QH₂). Step 4 is the chemical process of Q_bH^• releasing a hydrogen atom to generate Q, Q_bH^• → Q + H^•, and the thermodynamic capability is measured by the related Gibbs free energy, ΔG_HR(Q_bH^•). Step 5 is the chemical process of QH₂ successively releasing two hydrogen atoms via Step 1 and Step 2 to produce Q, QH₂ → Q + 2H^•, and the thermodynamic capability is measured by the related Gibbs free energy, ΔG_2HR(QH₂). Step 6 is the chemical process of QH₂ successively releasing two hydrogen atoms via Step 3 and Step 4 to produce Q, QH₂ → Q + 2H^•, and the thermodynamic capability is measured by the related Gibbs free energy, ΔG′_2HR(QH₂).

For the chemical reaction of asymmetric Q accepting two hydrogen atoms or H₂ to give QH₂, the reaction unfolds in seven discrete steps too (Steps 7–12), and the thermodynamic capabilities are quantified by the corresponding Gibbs free energy changes in Steps 7–12. Step 8 is the chemical process of Q accepting a hydrogen atom to generate Q_aH^• owning more substituents close to the resulting O-radical center, Q + H^• → Q_aH^•, and the thermodynamic capability is described as the corresponding Gibbs free energy, ΔG_HA(Q). Step 7 is the chemical process of Q_aH^• accepting a hydrogen atom to generate QH₂, Q_aH^• + H^• → QH₂, and the thermodynamic capability is described as the related Gibbs free energy, ΔG_HA(Q_aH^•). Step 10 is the chemical process of Q accepting a hydrogen atom to generate Q_bH^• owning less substituents close to the resulting O-radical center, Q + H^• → Q_bH^•, and the thermodynamic capability is measured by the corresponding Gibbs free energy, ΔG′_HA(Q). Step 9 is the chemical process of Q_bH^• accepting a hydrogen atom to generate QH₂, Q_bH^• + H^• → QH₂, and the thermodynamic capability is measured by the related Gibbs free energy, ΔG_HA(Q_bH^•). Step 11 is the chemical process of Q successively accepting two hydrogen atoms by Step 8 and Step 7 to produce QH₂, Q + 2H^• → QH₂, and the thermodynamic capability is measured by the related Gibbs free energy, ΔG_2HA(Q). Step 12 is the chemical process of Q successively accepting two hydrogen atoms by Step 11 and Step 10 to produce QH₂, Q + 2H^• → QH₂, and the thermodynamic capability is measured by the related Gibbs free energy, ΔG′_2HA(Q).

All the definitions on thermodynamic driving forces of 12 chemical processes for Q/QH₂ couples accepting or releasing two hydrogen atoms in DMSO are collected and shown in

Table 1. Definitions on driving forces of 12 chemical processes for Q/QH₂ couples accepting or releasing two hydrogen atoms or H₂ in DMSO.

Hydroquinones Releasing 2H• or H2			Quinones Accepting 2H• or H2
Step X	Definitions	Parameters	Step X	Definitions	Parameters
Step 1	QH2 → QaH• + H•	ΔGHR(QH2)	Step 7	QaH• + H• → QH2	ΔGHA(QaH•)
Step 2	QaH• → Q + H•	ΔGHR(QaH•)	Step 8	Q + H• → QaH•	ΔGHA(Q)
Step 3	QH2 → QbH• + H•	ΔG′HR(QH2)	Step 9	QbH• + H• → QH2	ΔGHA(QbH•)
Step 4	QbH• → Q + H•	ΔGHR(QbH•)	Step 10	Q + H• → QbH•	ΔG′HA(Q)
Step 5	QH2 → Q + 2H•	ΔG2HR(QH2)	Step 11	Q + 2H• → QH2	ΔG2HA(Q)
Step 6	QH2 → Q + 2H•	ΔG′2HR(QH2)	Step 12	Q + 2H• → QH2	ΔG′2HA(Q)

.

2.2. Calculation and Acquisition of Thermodynamic Data

ΔG_HR(QH₂) for Step 1 can be derived from the enthalpy change of QH₂ releasing a hydrogen atom to produce Q_aH^•, ΔH_HR(QH₂). The entropy change (TΔS) of QH₂ or QH^• releasing a hydrogen atom was estimated as 4.9 kcal/mol at 298 K in DMSO [29]; therefore, the ΔG_HR(QH₂) values in DMSO could be estimated from Equation (1), ΔG_HR(QH₂) = −ΔG_HA(Q_aH^•) = ΔH_HR(QH₂) − 4.9 kcal/mol [29]. The thermodynamic relation between Gibbs free energy and enthalpy change for hydrogen atoms transfer is verified many times in our previous work [30]. In addition, due to the fact that the chemical processes of QH₂ releasing a hydrogen atom to produce Q_aH^• and Q_aH^• accepting a hydrogen atom (Step 1: QH₂ → Q_aH^• + H^• and Step 7: Q_aH^• + H^• → QH₂) are opposite, the ΔG_HA(Q_aH^•) values for Step 7 could also be obtained by Equation (1), −ΔG_HA(Q_aH^•) = ΔG_HR(QH₂) = ΔH_HR(QH₂) − 4.9 kcal/mol. Similarly, the values of ΔG_HR(Q_aH^•) for Step 2 and ΔG_HA(Q) for Step 8 can be derived from Equation (2), ΔG_HR(Q_aH^•) = −ΔG_HA(Q) = ΔH_HR(Q_aH^•) − 4.9 kcal/mol. The values of ΔG′_HR(QH₂) for Step 3 and ΔG_HA(Q_bH^•) for Step 9 can be derived from Equation (3), ΔG′_HR(QH₂) = −ΔG_HA(Q_bH^•) = ΔH′_HR(QH₂) − 4.9 kcal/mol. The values of ΔG_HR(Q_bH^•) for Step 4 and ΔG′_HA(Q) for Step 10 can be derived from Equation (4), ΔG_HR(Q_bH^•) = −ΔG′_HA(Q) = ΔH_HR(Q_bH^•) − 4.9 kcal/mol. The details of DFT calculations of ΔH_HR(QH₂), ΔH_HR(Q_aH^•), ΔH′_HR(QH₂) and ΔH_HR(Q_bH^•) were available in our previous work [27].

ΔG_2HR(QH₂) for Step 5 can be obtained from Equation (5), ΔG_2HR(QH₂) = ΔG_HR(QH₂) + ΔG_HR(Q_aH^•), by constructing a thermodynamic cycle (Step 1–Step 2–Step 5) based on Hess’s law [28]. Similarly, ΔG_2HA(Q) for Step 11 can also be obtained from Equation (5), ΔG_2HR(QH₂) = −ΔG_2HA(Q) = ΔG_HR(QH₂) + ΔG_HR(Q_aH^•).

ΔG′_2HR(QH₂) for Step 6 can be obtained from Equation (6), ΔG′_2HR(QH₂) = ΔG′_HR(QH₂) + ΔG_HR(Q_bH^•), by constructing a thermodynamic cycle (Step 3−Step 4−Step 6) based on Hess’s law [28]. Similarly, ΔG_2HA(Q) for Step 13 can also be obtained from Equation (6), ΔG′_2HR(QH₂) = −ΔG′_2HA(Q) = ΔG′_HR(QH₂) + ΔG_HR(Q_bH^•).

All the expressions of Equations (1)−(6) and data sources on fourteen thermodynamic driving forces of Q/QH₂ couples accepting or releasing two hydrogen atoms in DMSO are illustrated in

Table 2. Expressions of Equations (1)~(6) and data sources on fourteen thermodynamic driving forces of quinones and hydroquinones accepting or releasing two hydrogen atoms in DMSO.

Equation X	Expressions	Data Sources
(1)	ΔGHR(QH2) = −ΔGHA(QaH•) = ΔHHR(QH2) − 4.9 kcal/mol	ΔHHR(QH2) were calculated by DFT [27]
(2)	ΔGHR(QaH•) = −ΔGHA(Q) = ΔHHR(QaH•) − 4.9 kcal/mol	ΔHHR(QaH•) were calculated by DFT [27]
(3)	ΔG′HR(QH2) = −ΔGHA(QbH•) = ΔH′HR(QH2) − 4.9 kcal/mol	ΔH′HR(QH2) were calculated by DFT [27]
(4)	ΔGHR(QbH•) = −ΔG′HA(Q) = ΔHHR(QbH•) − 4.9 kcal/mol	ΔHHR(QbH•) were calculated by DFT [27]
(5)	ΔG2HR(QH2) = −ΔG2HA(Q) = ΔGHR(QH2) + ΔGHR(QaH•)	derived in this work based on Hess’s law
(6)	ΔG′2HR(QH2) = −ΔG′2HA(Q) = ΔG′HR(QH2) + ΔGHR(QbH•)	derived in this work based on Hess’s law

.

3. Results

In our previous work [27], the values of ΔH_HR(QH₂) for Step 1, ΔH_HR(Q_aH^•) for Step 2, ΔH′_HR(QH₂) for Step 3, and ΔH_HR(Q_bH^•) for Step 4 of 118 important QH₂ and their intermediates Q_aH^• and Q_bH^• releasing a hydrogen atom in DMSO were calculated using DFT method. These original values of ΔH_HR(QH₂), ΔH_HR(Q_aH^•), ΔH′_HR(QH₂), and ΔH_HR(Q_bH^•) are displayed in Table S1 of Supplementary Materials.

Building upon this foundation, in this work, the values of ΔG_HR(QH₂) for Step 1, ΔG_HR(Q_aH^•) for Step 2, ΔG′_HR(QH₂) for Step 3, and ΔG_HR(Q_bH^•) for Step 4 of 118 important QH₂ and their intermediates, Q_aH^• and Q_bH^•, releasing a hydrogen atom, as well as the values of ΔG_HA(Q) for Step 7, ΔG_HA(Q_aH^•) values for Step 8, ΔG′_HA(Q) for Step 11, and ΔG_HA(Q_bH^•) for Step 12 of 118 Q and their intermediates Q_aH^• and Q_bH^• accepting a hydrogen atom in DMSO, were estimated by Equations (1)−(4), through considering the entropy change (TΔS) of QH₂ or QH^• releasing a hydrogen atom in DMSO (4.9 kcal/mol) [29,30,31].

In this work, ΔG_2HR(QH₂) for Step 5, and ΔG′_2HR(QH₂) for Step 6 of 118 QH₂ releasing two hydrogen atoms, as well as the values of ΔG_2HA(Q) for Step 9, and ΔG_2HA(Q) for Step 10 of 118 Q accepting two hydrogens in DMSO, were derived by Equations (5) and (6) based on Hess’s law.

A dataset encompassing all the thermodynamic results of 118 important Q/QH₂ couples accepting or releasing two hydrogen atoms about 12 chemical steps into DMSO is shown in

Table 3. Thermodynamic driving forces of 118 quinone/hydroquinone couples accepting or releasing two hydrogen atoms on 12 chemical steps in DMSO (unit: kcal/mol).

NO.	R	ΔGHR(QH2) Step 1	ΔG′HR(QH2) Step 3	ΔGHR(QaH•) Step 2	ΔGHR(QbH•) Step 4	ΔG2HR(QH2) Step 5	ΔG′2HR(QH2) Step 6
		−ΔGHA(QaH•) Step 8	−ΔGHA(QbH•) Step 12	−ΔGHA(Q) Step 7	−ΔG′HA(Q) Step 11	−ΔG2HA(Q) Step 9	−ΔG′2HA(Q) Step 10
1	H	78.5	−	64.5	−	143.0	−
2	N(Me)2	70.6	76.7	64.1	57.9	134.7	134.6
3	NH2	67.5	74.1	64.8	58.2	132.3	132.3
4	OMe	79.1	76.8	60.1	62.3	139.2	139.1
5	OH	73.2	78.0	65.4	60.6	138.6	138.6
6	SH	74.9	79.2	65.9	61.6	140.8	140.8
7	CH3	76.6	77.4	63.2	62.4	139.8	139.8
8	SiH3	76.9	79.0	65.8	63.6	142.7	142.6
9	F	78.0	79.5	66.0	64.5	144.0	144.0
10	Cl	79.8	78.2	63.9	65.5	143.7	143.7
11	Br	78.4	79.7	66.3	65.1	144.7	144.8
12	CHO	83.7	85.0	68.8	67.4	152.5	152.4
13	CO2Me	82.7	80.8	67.5	69.4	150.2	150.2
14	CF3	78.3	81.6	67.6	64.4	145.9	146.0
15	CN	78.8	82.4	69.4	65.9	148.2	148.3
16	OMe	76.9	−	64.4	−	141.3	−
17	CH3	75.4	−	61.0	−	136.4	−
18	F	78.8	−	65.1	−	143.9	−
19	Cl	79.1	−	64.6	−	143.7	−
20	CN	83.0	−	69.8	−	152.8	−
21	OMe	73.5	−	60.6	−	134.1	−
22	CH3	75.9	−	61.0	−	136.9	−
23	F	78.5	−	64.9	−	143.4	−
24	Cl	79.4	−	64.9	−	144.3	−
25	CN	82.1	−	69.0	−	151.1	−
26	OMe	80.3	79.4	54.1	55.0	134.4	134.4
27	CH3	75.1	77.5	61.8	59.4	136.9	136.9
28	F	70.0	73.6	67.1	63.4	137.1	137.0
29	Cl	81.1	77.6	63.1	66.6	144.2	144.2
30	CN	80.4	85.6	72.3	67.1	152.7	152.7
31	OMe	77.3	78.7	57.2	55.8	134.5	134.5
32	CH3	76.1	74.7	57.8	59.2	133.9	133.9
33	F	77.8	79.5	65.7	64.0	143.5	143.5
34	Cl	80.2	78.2	63.6	65.6	143.8	143.8
35	CN	83.6	85.5	72.2	70.2	155.8	155.7
36	OMe	74.9	−	60.0	−	134.9	−
37	CH3	74.0	−	57.0	−	131.0	−
38	F	78.7	−	65.0	−	143.7	−
39	Cl	79.2	−	64.2	−	143.4	−
40	CN	86.3	−	73.0	−	159.3	−
41	−	87.2	−	80.0	−	167.2	−
42	−	67.6	72.9	67.6	62.2	135.2	135.1
43	−	65.9	−	56.2	−	122.1	−
44	−	67.2	72.3	65.3	60.2	132.5	132.5
45	−	65.1	72.0	66.9	60.0	132.0	132.0
46	−	71.8	−	59.3	−	131.1	−
47	H	72.8	−	57.9	−	130.7	−
48	OMe	69.8	70.4	62.1	61.5	131.9	131.9
49	CH3	70.8	72.5	58.9	57.3	129.7	129.8
50	F	72.0	73.3	59.9	58.5	131.9	131.8
51	Cl	72.3	73.9	60.7	59.1	133.0	133.0
52	CN	72.8	77.0	64.4	60.3	137.2	137.3
53	−	70.0	−	58.1	−	128.1	−
54	OMe	70.1	67.6	52.1	54.7	122.2	122.3
55	CH3	68.2	69.7	56.9	55.5	125.1	125.2
56	F	69.5	70.2	57.3	56.7	126.8	126.9
57	Cl	69.6	71.4	59.0	57.2	128.6	128.6
58	CN	69.9	74.5	63.1	58.4	133.0	132.9
59	H	64.9	−	50.0	−	114.9	−
60	OMe	69.5	64.6	51.8	56.7	121.3	121.3
61	CH3	64.3	64.5	51.7	51.4	116.0	115.9
62	F	66.3	65.1	52.6	53.8	118.9	118.9
63	Cl	66.5	65.2	52.4	53.7	118.9	118.9
64	CN	64.4	65.9	53.5	51.9	117.9	117.8
65	OMe	63.7	63.8	51.3	51.2	115.0	115.0
66	CH3	64.3	64.4	51.7	51.6	116.0	116.0
67	F	65.7	65.4	52.3	52.6	118.0	118.0
68	Cl	65.5	65.6	52.8	52.7	118.3	118.3
69	CN	65.5	66.5	54.5	53.5	120.0	120.0
70	H	78.4	−	71.6	−	150.0	−
71	OMe	78.2	81.1	68.4	65.6	146.6	146.7
72	CH3	76.9	77.8	71.3	70.3	148.2	148.1
73	F	77.9	81.8	72.8	68.9	150.7	150.7
74	Cl	78.0	79.6	72.5	70.9	150.5	150.5
75	CN	78.0	79.5	73.7	72.3	151.7	151.8
76	OMe	74.9	72.8	66.8	68.9	141.7	141.7
77	CH3	77.2	75.9	69.8	71.0	147.0	146.9
78	F	78.0	77.1	70.7	71.5	148.7	148.6
79	Cl	79.0	78.1	71.3	72.2	150.3	150.3
80	CN	81.9	81.8	75.0	75.0	156.9	156.8
81	OMe	78.5	76.9	66.6	68.1	145.1	145.0
82	CH3	77.1	76.6	69.6	70.1	146.7	146.7
83	F	77.9	78.4	71.9	71.4	149.8	149.8
84	Cl	78.2	79.0	72.0	71.2	150.2	150.2
85	CN	81.6	82.8	77.5	76.4	159.1	159.2
86	OMe	73.9	79.9	65.1	59.1	139.0	139.0
87	CH3	76.1	77.0	69.5	68.6	145.6	145.6
88	F	76.8	79.9	72.9	69.8	149.7	149.7
89	Cl	77.4	80.0	73.2	70.6	150.6	150.6
90	CN	81.3	82.7	76.8	75.4	158.1	158.1
91	OMe	80.1	−	65.5	−	145.6	−
92	CH3	75.7	−	70.3	−	146.0	−
93	F	79.1	−	72.5	−	151.6	−
94	Cl	79.1	−	72.0	−	151.1	−
95	CN	80.6	−	74.5	−	155.1	−
96	OMe	77.2	−	55.2	−	132.4	−
97	CH3	75.1	−	69.5	−	144.6	−
98	F	77.2	−	70.3	−	147.5	−
99	Cl	78.8	−	72.5	−	151.3	−
100	CN	84.8	−	77.6	−	162.4	−
101	OMe	78.2	74.6	65.7	69.2	143.9	143.8
102	CH3	75.2	73.8	68.4	69.9	143.6	143.7
103	F	79.7	78.0	71.2	72.8	150.9	150.8
104	Cl	79.4	78.4	71.3	72.2	150.7	150.6
105	CN	83.9	83.7	77.8	78.0	161.7	161.7
106	OMe	76.0	77.7	60.5	58.8	136.5	136.5
107	CH3	73.9	74.4	69.2	68.7	143.1	143.1
108	F	76.8	78.7	71.9	70.0	148.7	148.7
109	Cl	78.2	80.0	73.2	71.4	151.4	151.4
110	CN	84.4	85.5	79.9	78.9	164.3	164.4
111	OMe	73.7	−	64.4	−	138.1	−
112	CH3	73.7	−	68.7	−	142.4	−
113	F	78.3	−	71.6	−	149.9	−
114	Cl	79.0	−	72.2	−	151.2	−
115	CN	86.4	−	81.0	−	167.4	−
116	−	71.1	73.1	66.3	64.3	137.4	137.4
117	−	78.9	−	81.3	−	160.2	−
118	−	68.6	−	62.3	−	130.9	−

.

4. Discussion

4.1. Further Verification of Data Reliability

The reliability of calculated enthalpy changes in a hydrogen atom release from QH₂ and QH^•, including ΔH_HR(QH₂) for Step 1, ΔH_HR(Q_aH^•) for Step 2, ΔH′_HR(QH₂) for Step 3, and ΔH_HR(Q_bH^•) for Step 4, was verified in our previous work [27]. For 33 substituted phenols, the theoretically predicted enthalpy changes in O−H homolysis fit well with the experimental data (MD = 0.58 and r = 0.98) [27].

In this work, the data accuracy is further verified. For the chemical process of QH₂ releasing two hydrogen atoms, two distinct pathways are involved. Pathway 1 (Step 5) is when QH₂ successively releases two hydrogen atoms by Step 1 and Step 2; Step 1 QH₂ → Q_aH^• + H^•, and Step 2 Q_aH^• → Q + H^•. In contrast, pathway 2 (Step 6) is when QH₂ successively releases two hydrogen atoms by Step 3 and Step 4; Step 3 QH₂ → Q_bH^• + H^•, and Step 4 Q_bH^• → Q + H^•. Theoretically, if the calculated thermodynamic data is reliable, the energy changes in pathways 1 (Step 5) and 2 (Step 6) should ideally be the same based on Hess’s law [28]. The energy difference between ΔG_2HR(QH₂) for Step 5 and ΔG′_2HR(QH₂) for Step 6 is denoted as ΔΔG_2HR, ΔΔG_2HR = ΔG′_2HR(QH₂) − ΔG_2HR(QH₂), which are listed in the first column of Table S2 in Supplementary Materials. As expected, the range of ΔΔG_2HR is found to be between −0.1 to 0.1 kcal/mol. By inference, the extremely slight difference in energy strongly suggests that the calculated energies for O–H homolysis are indeed reliable, and so are these derived thermodynamic results of the 12 chemical steps (Steps 1–12) in this work.

4.2. Thermodynamic Capabilities of QH₂ Acting as Antioxidants by Releasing Two Hydrogen Atoms

Hydroquinones possess a unique antioxidant characteristic [10] that sets them apart from conventional antioxidants, such as butylated hydroxytoluene (BHT) [31] and α-tocopherol (TocOH) [31], as well as nicotinamide coenzymes [NAD(P)H] [28] and so on. Unlike these substances, a single hydroquinone molecule has the capacity to sequentially release two hydrogen atoms, thereby neutralizing two radicals in the process as antioxidants. The same antioxidant characteristic occurs in ascorbic acid (AscH₂) [31], coenzyme Q [1,2,3,4], and hantzsch ester (HEH₂) [30], all of which are capable of donating two hydrogen atoms during their antioxidant activity. Given this distinctive feature, the thermodynamic data of QH₂ releasing two hydrogen atoms, ΔG_2HR(QH₂), becomes a crucial parameter to evaluate their overall antioxidant properties.

Herein, the Gibbs free energies for the sequential release of two hydrogen atoms from hydroquinones and other common reductants in DMSO are presented in Scheme 3 for comparative analysis. A visual examination of Scheme 3 reveals that the ΔG_2HR(QH₂) scale of 118 QH₂ spans from 114.9 kcal/mol to 167.4 kcal/mol in DMSO. In particular, for the 69 Q_pH₂ (1H₂–69H₂), the ΔG_2HR(Q_pH₂) scale spans from 114.9 kcal/mol to 167.2 kcal/mol, while for the 49 Q_oH₂ (70H₂–118H₂), the ΔG_2HR(Q_oH₂) scale spans from 130.9 kcal/mol to 167.4 kcal/mol. It is observed that the Q_pH₂ (114.9–167.2 kcal/mol) demonstrate a broader thermodynamic window for two-hydrogen-atoms release, compared with Q_oH₂ (130.9–167.4 kcal/mol). Notably, both Q_pH₂ (114.9–167.2 kcal/mol) and Q_oH₂ (130.9–167.4 kcal/mol) share a similar thermodynamic upper limit for two-hydrogen-atoms release in DMSO, capped at approximately 167 kcal/mol. Moreover, 19 Q_pH₂ (114.9–167.2 kcal/mol), including 43H₂ (122.1 kcal/mol), 47H₂ (130.7 kcal/mol), 49H₂ (129.7 kcal/mol), 53H₂–57H₂ (122.2–128.6 kcal/mol), and 59H₂–69H₂ (114.9–121.3 kcal/mol) have the better thermodynamic capabilities to release two hydrogen atoms when compared to the entire set of 49 Q_oH₂ (70H₂–118H₂, 130.9–167.4 kcal/mol) in DMSO. What is more, among these 118 QH₂, the 9,10-hydroanthraquinones (59H₂–69H₂, 114.9–121.3 kcal/mol) stand out as the most thermodynamically favorable two-hydrogen-atoms antioxidants.

HEH₂, AscH₂ and coenzyme Q are recognized for their efficacy as antioxidants capable of donating two hydrogen atoms [30,31], and the Gibbs free energies of HEH₂ (117.9 kcal/mol) [32,33,34,35,36], AscH₂ (133.6 kcal/mol in H₂O) [31] and the close coenzyme Q model 2,3-Me₂-5,6-MeO₂-p-hydroquinone (CoQH₂, 133.0 kcal/mol) are displayed in Scheme 3 for comparative purposes. In addition, tetracyano-p-benzoquinone (40H₂) is noted for its exceptional oxidizing properties, with a ΔG_2HR(40H₂) value of 159.3 kcal/mol in DMSO. Based on these references, a classification system can be inferred for the thermodynamic strength of QH₂ acting as two-hydrogen-atoms antioxidants. If a ΔG_2HR(QH₂) value is less than or equal to 130 kcal/mol, the QH₂ is categorized as a thermodynamically strong two-hydrogen-atoms antioxidant. If a ΔG_2HR(QH₂) value is greater than 130 kcal/mol but less than or equal to 150 kcal/mol, the QH₂ is considered a thermodynamically medium–strong two-hydrogen-atoms antioxidant. If a ΔG_2HR(QH₂) value exceeds 150 kcal/mol, the QH₂ is clarified as a thermodynamically weak two-hydrogen-atoms antioxidant. This categorization provides a clear framework for assessing the overall antioxidant potential based on the thermodynamic data of two-hydrogen-atoms donation, offering valuable insights for the design and evaluation of novel antioxidants.

Accordingly, all the 118 QH₂ (114.9–167.4 kcal/mol) and 69 Q_pH₂ (114.9–167.2 kcal/mol) cover from the thermodynamically strong, medium–strong, to weak two-hydrogen-atoms antioxidants. The 49 Q_oH₂ are recognized as the thermodynamically medium-strong or weak two-hydrogen-atoms antioxidants. Upon closer examination, 19 Q_pH₂ (114.9–167.2 kcal/mol), including 43H₂ (122.1 kcal/mol), 47H₂ (130.7 kcal/mol), 49H₂ (129.7 kcal/mol), 53H₂–57H₂ (122.2–128.6 kcal/mol), and 59H₂–69H₂ (114.9–121.3 kcal/mol), are recognized as the thermodynamically strong two-hydrogen-atoms antioxidants. The 30 QH₂ (>150 kcal/mol), including 8 Q_pH₂ and 22 Q_oH₂, that is, 12H₂ (152.5 kcal/mol), 13H₂ (150.2 kcal/mol), 20H₂ (152.8 kcal/mol), 25H₂ (151.1 kcal/mol), 30H₂ (152.7 kcal/mol), 35H₂ (155.8 kcal/mol), 40H₂ (159.3 kcal/mol), 41H₂ (167.2 kcal/mol), 73H₂–75H₂ (150.5–151.7 kcal/mol), 79H₂ (150.3 kcal/mol), 80H₂ (156.9 kcal/mol), 84H₂ (150.2 kcal/mol), 85H₂ (159.1 kcal/mol), 89H₂ (150.6 kcal/mol), 90H₂ (158.1 kcal/mol), 93H₂–95H₂ (151.1–155.1 kcal/mol), 99H₂ (151.3 kcal/mol), 100H₂ (162.4 kcal/mol), 103H₂–105H₂ (150.7–161.7 kcal/mol), 109H₂ (151.4 kcal/mol), 110H₂ (164.3 kcal/mol), 114H₂ (151.2 kcal/mol), 115H₂ (167.4 kcal/mol) and 117H₂ (160.2 kcal/mol), are identified as thermodynamically weak two-hydrogen-atoms antioxidants with ΔG_2HR(QH₂) values exceeding 150 kcal/mol. The remaining 70 QH₂ fall into the category of thermodynamically medium–strong two-hydrogen-atoms antioxidants. Among all 118 QH₂, 59H₂ (9,10-hydroanthraquinone, 114.9 kcal/mol) is recognized as the most thermodynamically favorable two-hydrogen-atoms antioxidant [10]. In contrast, 115H₂, an o-hydroquinone with four strong electron-withdrawing cyano groups (167.4 kcal/mol), exhibits the highest ΔG_2HR value of 167.4 kcal/mol, marking it as the thermodynamically weakest two-hydrogen-atoms antioxidant.

Intriguingly, just like HEH₂ (117.9 kcal/mol) [35,36], 9,10-hydroanthraquinones (59H₂–69H₂, 114.9–121.3 kcal/mol) exhibit the similar thermodynamic capabilities to release two hydrogen atoms. This similarity in thermodynamic capability to release two hydrogen atoms suggests that 9,10-hydroanthraquinones (59H₂–69H₂) could serve as viable alternatives to HEH₂ (117.9 kcal/mol) as hydrogen reducers or two-hydrogen-atoms donors in chemical reactions. In particular, 59H₂ (114.9 kcal/mol), 61H₂ (116.0 kcal/mol), 65H₂ (115.0 kcal/mol), and 66H₂ (116.0 kcal/mol), are identified as thermodynamically better two-hydrogen-atoms antioxidants or reducers than HEH₂ (117.9 kcal/mol) in DMSO. These findings underscore and reveal the potential utility of 9,10-hydroanthraquinones as robust antioxidants or reducers in chemical area [10,17,18,19,20,21], particularly in the context of reactions involving DMSO as a solvent.

Scheme 3. Gibbs free energies of hydroquinones and common reductants successively releasing two hydrogen atoms in DMSO.

Scheme 3. Gibbs free energies of hydroquinones and common reductants successively releasing two hydrogen atoms in DMSO.

4.3. Thermodynamic Capabilities of QH₂ and QH^• Releasing a Hydrogen Atom as Antioxidants

As previously discussed, the ΔG_2HR(QH₂) values reflect the overall antioxidant properties of QH₂ when releasing two hydrogen atoms. For a given QH₂, especially for the asymmetric hydroquinones, it is widely known that there exists a huge difference in the thermodynamic capabilities between QH₂ and QH^• releasing a hydrogen atom due to the different levels of stability of QH₂ and its active intermediate QH^•. Therefore, the thermodynamic capabilities of QH₂ and QH^• releasing a hydrogen atom, ΔG_HR(QH₂) and ΔG_HR(QH^•), are also the crucial thermodynamic parameters for assessing their separate antioxidant potentials of QH₂ and QH^•. To delve deeper into these properties, Scheme 4 presents the Gibbs free energies associated with the release of a hydrogen atom from QH₂, QH^•, and various O–H bonds in DMSO. By examining these Gibbs free energies, researchers can gain insights into the relative reactivity and stability of QH₂ and QH^•, which is essential for understanding their roles in antioxidant processes and for the design of novel antioxidant compounds [10].

As reported in our previous work, [28] a criterion was established for classifying the thermodynamic antioxidant capability. If a ΔG_HR value is less than or equal to 65 kcal/mol, the QH₂ or QH^• belongs to a thermodynamically strong antioxidant. If a ΔG_HR value is greater than 65 kcal/mol and less than or equal to 85 kcal/mol, the QH₂ or QH^• belongs to a thermodynamically medium–strong antioxidant. If a ΔG_2HR(QH₂) value exceeds 85 kcal/mol, the QH₂ is classified a thermodynamically weak antioxidant.

From Scheme 4, for the first hydrogen atom release, it is clear that the ΔG_HR(QH₂) values of 118 QH₂ range from 63.7 to 87.2 kcal/mol, which spans a large scope of 23.5 kcal/mol. To be specific, for p-hydroquinones, the ΔG_HR(Q_pH₂) values of 69 Q_pH₂ (1H₂–69H₂) also range from 63.7 to 87.2 kcal/mol. Meanwhile, as for o-hydroquinones, the ΔG_HR(Q_oH₂) values of 49 Q_oH₂ (70H₂–118H₂) range from 68.6 to 86.4 kcal/mol. The ΔG_HR(Q_pH₂) values (63.7–87.2 kcal/mol) demonstrate a wider thermodynamic spread compared to the ΔG_HR(Q_oH₂) values (68.6–86.4 kcal/mol). These findings suggest that QH₂ (63.7–87.2 kcal/mol) are generally considered as the thermodynamically medium–strong antioxidants.

Upon examining the second hydrogen atom release, the ΔG_HR(QH^•) values of 118 QH^• exhibit a substantial range, from 50.0 to 81.3 kcal/mol, which spans an extensive thermodynamic scope of 31.3 kcal/mol. Specifically, for p-hydroquinones, the ΔG_HR(Q_pH^•) values of 69 Q_pH^• (1H^•–69H^•) range from 50.0 to 80.0 kcal/mol. In comparison, for o-hydroquinones, the ΔG_HR(Q_oH^•) values of 49 Q_oH^• (70H^•–118H^•) range from 55.2 to 81.3 kcal/mol. Similarly, the ΔG_HR(Q_pH^•) values (50.0–80.0 kcal/mol) demonstrate a wider thermodynamic distribution than ΔG_HR(Q_oH^•) values (55.2–81.3 kcal/mol). The thermodynamic results suggest that QH^• (50.0–81.3 kcal/mol) are clarified as the thermodynamically strong and medium–strong antioxidants. Notably, the thermodynamic antioxidant capabilities of QH^• (50.0–81.3 kcal/mol) appear to be enhanced relative to their parent compounds QH₂, which have ΔG_HR(QH₂) values ranging from 63.7 to 87.2 kcal/mol.

Scheme 4. Gibbs free energies of QH₂, QH^•, and various X–H bonds releasing a hydrogen atom in DMSO.

Scheme 4. Gibbs free energies of QH₂, QH^•, and various X–H bonds releasing a hydrogen atom in DMSO.

Further considering the thermodynamic capabilities of 9,10−hydroanthraquinones and their corresponding semiquinone radicals, 59H₂–69H₂ and 59H^•–69H^• are generally thermodynamically strong antioxidants with the thermodynamic driving forces of QH₂ and QH^• releasing a hydrogen atom smaller than 65 kcal/mol. Except for the Gibbs free energies of 9,10-hydroanthraquinones (59H₂–69H₂) releasing a hydrogen atom, the ΔG_HR(QH₂) values of the other 107 QH₂ exceeds 65 kcal/mol, indicating that these 107 QH₂ belong to thermodynamically medium–strong and weak antioxidants during the first hydrogen atom release from their hydroquinone states.

For a given QH₂, if the ΔG_HR(QH₂) and ΔG′_HR(QH₂) values are compared with its ΔG_HR(Q_aH^•) and ΔG_HR(Q_aH^•) values, respectively, the energy differences between them are defined as ΔΔG_HR and ΔΔG′_HR, ΔΔG_HR = ΔG_HR(Q_aH^•) − ΔG_HR(QH₂) and ΔΔG′_HR = ΔG_HR(Q_bH^•) − ΔG′_HR(QH₂), which are presented in the fourth and fifth columns of Table S2 in Supplementary Materials. The ΔΔG_HR and ΔΔG″_HR scales could provide insight into the thermodynamic favorability and tendency of each step. [30] It is discovered that the ΔΔG_HR and ΔΔG″_HR scales range from −26.2 kcal/mol for 26H₂ to 2.4 kcal/mol for 117H₂, from −24.4 kcal/mol for 26H₂ to −3.9 kcal/mol for 76H₂ and 102H₂ respectively. Except for 42H₂ (0.0 kcal/mol) and 117H₂ (2.4 kcal/mol), the second hydrogen atom release from QH^• is thermodynamically more favorable than the first hydrogen atom release from their parents QH₂. Furthermore, the majority of ΔΔG_HR and ΔΔG′_HR values are more negative than −5 kcal/mol, meaning that after QH₂ releases a hydrogen atom, the following hydrogen atom transfer from generated QH^• has greater thermodynamic driving forces.

In addition, for an asymmetric QH₂, if the ΔG_HR(QH₂) values for Step 1 and ΔG′_HR(QH₂) values for Step 3 are compared, the energy difference between them is denoted as ΔΔG″_HR, ΔΔG″_HR = ΔG′_HR(QH₂) − ΔG_HR(QH₂), which are detailed in the sixth column of Table S2 in Supplementary Materials. The ΔΔG″_HR scale ranges from −4.9 kcal/mol for 60H₂ to 6.9 kcal/mol for 45H₂. This range indicates that the preference for an asymmetric QH₂ to release a hydrogen atom to form either the semiquinone radical with more substituents adjacent to the nascent O-radical center (Q_aH^•) or the one with fewer substituents (Q_bH^•) is influenced by the stability differences between Q_aH^• and Q_bH^•, which are attributed to the effects of the substituents. This finding highlights the importance of considering the molecular environment surrounding the reactive centers when predicting the antioxidant behavior of hydroquinones and their derived semiquinone radicals.

4.4. Thermodynamic Capabilities of QH₂ and QH^• Acting as Radical Quenchers

In chemical reactions, HEH₂ were used to quench the active radicals (R^•) by donating two hydrogen atoms [35,36], resulting in the formation of stable RH. Given that QH₂ and QH^• are capable of releasing hydrogen atom(s) as antioxidants, they can similarly serve as radical quenchers in radical reactions. Hence, Scheme 5 displays the Gibbs free energies for the release of hydrogen atom(s) from hydroquinones, semiquinones (QH^•), and common organic compounds (RH) [31,37,38,39] in DMSO to provide a comparative analysis of their thermodynamic feasibility as radical quenchers. By examining these Gibbs free energies from Scheme 5, one can assess and deduce the relative propensity of these compounds to engage in hydrogen atom transfer reactions, which is crucial for their effectiveness in radical quenching roles.

As depicted in Scheme 5, the Gibbs free energies of RH releasing a hydrogen atom range from 76.0 kcal/mol for C_sp3–H bond in DHA (dihydroanthracene) [31] to 128.1 kcal/mol for C_sp–H bond in acetylene [37,38,39]. Since the ΔG_HR(QH₂) and ΔG_HR(QH^•) values of 118 QH₂ and QH^• range from 63.7 to 87.2 kcal/mol and 50.0 to 81.3 kcal/mol, respectively, this indicates that the majority of QH₂ and QH^• species examined in this work are capable of quenching a broad spectrum of active radicals (R^• and 76.0–128.1 kcal/mol) in chemical reactions. Since QH₂ can release two hydrogen atoms, the Gibbs free energies of two moles R^• accepting two moles hydrogen atoms, ΔG_2HA(R^•), therefore, are estimated to range from 152.0 kcal/mol to 256.2 kcal/mol in DMSO. As discussed and shown in Scheme 5b, the ΔG_2HR(QH₂) scale of 118 QH₂ ranges from 114.9 kcal/mol to 167.4 kcal/mol in DMSO, suggesting that 104 QH₂ with ΔG_2HR(QH₂) less than 152 kcal/mol are well-suited to quench nearly all the active radicals within the 152.0 to 256.2 kcal/mol range through the two-hydrogen-atoms transfer reaction, QH₂ + 2R^• → Q + 2R−H, driven by a significant negative thermodynamic driving force (ΔG << 0 kcal/mol). What is even more interesting is that although certain oxidants such as 115H₂ (tetracyano-o-benzoquinone, 167.4 kcal/mol) and 40H₂ (tetracyano-p-benzoquinone, 159.3 kcal/mol) are thermodynamically very weak two-hydrogen-atoms donors, they still possess the potential to act as radical quenchers. They are capable of neutralizing a variety of active radicals, including, but not limited to, PhCH₂^• (87 kcal/mol) [31], O-radicals (174.0–122.7 kcal/mol), Ph^• (108.0 kcal/mol) [37,38,39], and HC≡C^• (128.1 kcal/mol) [37,38,39], through the release of two hydrogen atoms. These findings reveal the versatility of QH₂ and QH^• as antioxidants and radical quenchers in chemical reactions, highlighting their thermodynamic potential to engage in hydrogen atom transfer (HAT) processes.

Scheme 5. (a) Gibbs free energies of hydroquinones and semiquinones releasing hydrogen atoms, along with Gibbs free energies of common organic compounds (RH) releasing hydrogen atoms. (b) Gibbs free energies of hydroquinones successively releasing two hydrogen atoms in DMSO.

Scheme 5. (a) Gibbs free energies of hydroquinones and semiquinones releasing hydrogen atoms, along with Gibbs free energies of common organic compounds (RH) releasing hydrogen atoms. (b) Gibbs free energies of hydroquinones successively releasing two hydrogen atoms in DMSO.

4.5. Thermodynamic Capabilities of Q and QH^• Acting as Hydrogen Atoms Abstractors

QH₂ and QH^• can function as antioxidants or radical quenchers by releasing hydrogen atom(s), and conversely, their oxidized forms, Q and QH^•, can act as hydrogen atoms abstractors to initiate radical reactions. In this context, Scheme 6 compiles the Gibbs free energies of quinones (Q) and semiquinones (QH^•) accepting hydrogen atoms, as well as common free radicals accepting hydrogen atoms in DMSO.

The capability of certain compounds to act as efficient hydrogen atom abstractors in chemical reactions is characterized by their hydrogen atom affinities (Gibbs free energies of compounds accepting a hydrogen atom). ^tBuO^• (−106.0 kcal/mol) [31,40,41,42,43] and PINO (phthalimide-N-oxyl radical, −87.0 kcal/mol) [31,44,45] are efficient hydrogen atoms abstractors in chemical reactions. If the hydrogen atom affinities of Q or QH^•, ΔG_HA(Q) or ΔG_HA(QH^•) are more negative than −85 kcal/mol, the Q or QH^• are recognized as thermodynamically strong hydrogens atoms abstractors. Since TEMPO (−67.5 kcal/mol) [31,46,47,48,49] and PhS^• (−76.9 kcal/mol) [31,50,51,52] have been reported as medium–strong radical initiators by abstracting active hydrogen atoms, it seems reasonable to define a thermodynamically medium–strong category for hydrogen atom abstractors. Accordingly, if the hydrogen atom affinities of Q and QH^• [ΔG_HA(Q) and ΔG_HA(QH^•)] fall between −65 kcal/mol and −85 kcal/mol, the corresponding Q or QH^• are recognized as thermodynamically medium–strong hydrogens atoms abstractors. Finally, if the ΔG_HA(Q) or ΔG_HA(QH^•) values are more positive than −65 kcal/mol, the Q or QH^• are regarded as thermodynamically weak hydrogens atoms abstractors.

Scheme 6. Gibbs free energies of quinones (Q) and semiquinones (QH^•) accepting hydrogen atoms, as well as common free radicals accepting hydrogen atoms in DMSO.

Scheme 6. Gibbs free energies of quinones (Q) and semiquinones (QH^•) accepting hydrogen atoms, as well as common free radicals accepting hydrogen atoms in DMSO.

This classification system provides a clear framework for evaluating the thermodynamic potential of Q and QH^• to initiate radical reactions through hydrogen atom abstraction. Because the ΔG_HA(Q) (−81.3–−50.0 kcal/mol) and ΔG_HA(QH^•) (−87.2–−63.7 kcal/mol) values are more positive than −87.2 kcal/mol, the thermodynamically hydrogen atoms abstracting capabilities of Q and QH^• are considerably weaker compared to efficient abstractors like ^tBuO^• (−106.0 kcal/mol) [31,40,41,42,43]. For quinones, the ΔG_HA(Q) values range from −81.3 kcal/mol to −50.0 kcal/mol, indicating that 118 quinones fall into the category of thermodynamically weak or medium–strong hydrogen atom abstractors. This suggests a need for careful selection to identify suitable hydrogen atom abstractors. Additionally, DPPH (2,2-diphenyl-1-picrylhydrazyl, −80.0 kcal/mol) [53,54,55,56,57,58] is employed as an important reference material to determine the antioxidant activity of substances by absorbing a hydrogen atom. From a thermodynamic perspective, certain quinones like 41 (−80.0 kcal/mol), 110 (−79.9 kcal/mol), 115 (−81.0 kcal/mol) and 117 (−81.3 kcal/mol) exhibit potential to be used as the alternatives to DPPH in antioxidant determination. Most notably, the ΔG_2HA(Q) values of 41 (−167.2 kcal/mol), 110 (−164.3 kcal/mol), 115 (−167.4 kcal/mol) and 117 (−160.2 kcal/mol) are known in this work; therefore, 41, 110, 115 and 117 could also serve as reference materials to determine the BDFE (bond dissociation free energy) of X–H in antioxidant or chemical substances through direct calorimetry.

In contrast to quinones, semiquinone radicals (QH^•) exhibit an extensive range of thermodynamic capabilities to abstract a hydrogen atom, as indicated by their ΔG_HA(QH^•) values, which vary from −87.2 kcal/mol to −63.7 kcal/mol. This range encompasses thermodynamically weak, medium–strong, and strong hydrogen atom abstractors. Five semi-9,10-anthraquinone radicals (−63.7–−64.9 kcal/mol), including 59H^•, 61H^•, 64H^•, 65H^• and 66H^•, are clarified as the thermodynamically weak hydrogen atom abstractors and may not be suitable for use as such in chemical reactions. On the other end of the spectrum, 40H^• (−86.3 kcal/mol), 41H^• (−87.2 kcal/mol), 115H^• (−86.4 kcal/mol), and 110_bH^• (−85.5 kcal/mol) are considered thermodynamically strong hydrogen atoms abstractors, which could be used as the alternatives to PINO (−87.0 kcal/mol) [31,44,45] for initiating radical reactions through hydrogen atom abstraction. In summary, the majority of QH^• (109 out of 118) are categorized as thermodynamically medium–strong hydrogen atom abstractors, which hold the potential to be used as alternatives to TEMPO (−67.5 kcal/mol) [31,46,47,48,49] or PhS^• (−76.9 kcal/mol) [31,50,51,52] for abstracting a hydrogen atom and initiating radical reactions, offering a broad spectrum of applications in the realm of redox chemistry.

5. Conclusions

In summary, this work presents a practical thermodynamic dataset that encapsulates the thermodynamic driving forces for 12 distinct chemical processes involving the hydrogenation and dehydrogenation of 118 quinone/hydroquinone couples in DMSO. This valuable dataset is designed for convenient access and practical utility by researchers in the field. Based on the thermodynamic dataset, the thermodynamic capabilities of hydroquinone acting as two-hydrogen-atoms antioxidants and radical quenchers, hydroquinone and semiquinone radicals acting as hydrogen atoms abstractors, and quinone/hydroquinone couples acting as dehydrogenation and hydrogenation reagents, are discussed and explored in detail. The findings and insights are expected to provide a deep understanding of fundamental thermodynamics for quinone/hydroquinone couples in solution, which is instrumental in advancing the extensive application across various fields, including chemical antioxidation and redox reactions.