Ultraviolet Absorption Spectra of Benzene and Chlorobenzene in Water-Ice Solutions at Temperatures Between 78 K and 273 K

Abstract

In this paper, characteristic ultraviolet absorption spectra are presented for benzene and chlorobenzene in transparent hexagonal water–ice solutions at temperatures between 273 K and 78 K. In addition, the liquid solution spectra at 292 K have also been included. The two lowest symmetry-forbidden transitions from the ground state (¹A_1g) to the first excited level of symmetry (B_2u), denoted as ¹B_2u ← ¹A_1g, and the transition from the ground state to the second excited level of symmetry (¹B_1u), denoted as ¹B_1u ← ¹A_1g, of benzene are recorded. The two lowest transitions of chlorobenzene from the ground state (¹A₁) to the first excited level of symmetry (¹B₂), denoted as ¹B₂ ← ¹A₁, and the transition from the ground state to the second excited level of symmetry (¹A₁) denoted as, ¹A₁ ← ¹A₁, are also studied. The bands are obtained for slowly cooled transparent water–ice solutions. Such ice samples, that were frozen from liquid water and cooled, show gradual changes in the spectra. Our study shows the spectra at eight temperatures, separating the spectra in different regions based on the range for the bands from ground state to the first and second excited states of benzene and chlorobenzene, observing changes in the integrated absorbances as a function of the temperature. For the spectra recorded at 78 K, the peak absorbances as a function of the wavelength are presented and tentatively assigned. Peak assignments are based on the known literature of benzene and chlorobenzene. The temperature range of our study covers some of the average temperatures that have been found in the icy moons of Saturn and the polar regions of Earth.

1. Introduction

Previous low temperature studies of solid [1], solid solutions [2,3], liquid solutions [4,5] and room temperature gas phase [6,7,8,9,10] of the vacuum UV and UV absorption bands of benzene, separate the spectra in three regions. A strong transition from the ground state of symmetry (¹A_1g) to the third singlet state (¹E_1u), denoted as ¹E_1u $\leftarrow$ ¹A_1g. This symmetry allowed strong absorption band peaks around 180 nm [1,2]. Near the base of the strong band there is small shoulder from the ground state to the second excited singlet state (¹$B_{1u}$), denoted as ¹$B_{1u}\leftarrow$ ¹$A_{1g}$. This transition is symmetry forbidden but appears because of Herzberg-Teller vibronic coupling and by resonance borrows intensity from the allowed band showing vibronic structure. Another transition, the weakest of the three, is from the ground state to the first excited singlet (¹$B_{2u}$) denoted as ¹$B_{2u}\leftarrow$ ¹$A_{1g}$. It is also symmetry forbidden but appears due to vibronic coupling and intensity borrowing [1,2].

If one hydrogen atom of benzene is substituted by a chlorine atom, the symmetry is lowered from D_6h, to C_2v, and the degeneracy of the highest occupied π orbitals is lost. Due to the C_2v symmetry, the ¹B_1u and ¹B_2u states of benzene are replaced by the ¹A₁ and ¹B₂ in chlorobenzene [11,12,13,14]. The ground state has symmetry ¹$A_1$ and the transitions then are the transition to the second excited state, denoted as ¹$A_1\leftarrow$ ¹$A_1$, and the transition to the first excited state, denoted as ¹$B_2\leftarrow$ ¹$A_1$. The ultraviolet (UV) spectra of benzene and chlorobenzene are distinct mainly in the location and strength of their absorption bands. The spectral variations occur because the chlorine atom in chlorobenzene can interact with the benzene ring via its lone-pair electrons. Although chlorine can participate in resonance, its inductive effect is more pronounced, resulting in the withdrawal of electron density from the ring. The interaction reduces the energy needed for excitation therefore chlorobenzene displays a red shift with respect to benzene.

Our study of benzene and chlorobenzene shows the spectra at eight temperatures and one concentration. The spectra are separated in different regions based on the range of the bands for the first excited and second excited transitions. Changes in the integrated absorbances as a function of the temperature are reported. The effects of the water–ice matrix on the magnitude of the integrated intensities are discussed. For the spectra recorded at 78 K, the peak absorbances as a function of the wavelength are presented and tentatively assigned. Peak assignments are based on the known literature of vibrational levels of benzene [15] and chlorobenzene [16,17,18,19,20].

The temperature range of our study covers some of the average temperatures that have been found in the icy moons of Saturn and the polar regions of Earth. Among the molecules that have been proposed as trapped or attached to water ice in the solar system, benzene has been mentioned on Saturn’s moon Enceladus. The temperature range of our study covers some of the average temperatures that have been reported [21]. Chlorobenzene has only been mentioned on the surface of Mars, but its presence has been questioned due to the pyrolysis conducted before the analysis of the sample [22].

It has been found that the photolysis of chlorobenzene in water ice with wavelengths greater than 254 nm, produces biphenyl and terphenyl as well as their chlorinated isomers [23] while the photochemistry in water produces mainly phenol derivatives [24]. On earth chlorobenzene is produced in industry and can find its way into the atmosphere and then precipitate at the poles. Combustion of chlorinated benzenes can generate polychlorinated biphenyls (PCBs) [25] and tetrachlorodibenzo-1,4-dioxin [26] that are very toxic for the environment. Measurements of Earth polar environments and mountains near the poles made in deeper snow layers and glacial ice cores, show chemicals that include PCBs and organochlorine (OC) pesticides [27].

2. Materials and Methods

2.1. Materials

The chemicals utilized in the experiment were employed without any additional purification. Benzene (99%, EMD Millipore, Darmstadt, Germany) and Chlorobenzene (99%, Alfa Aesar, Heysham, UK) were acquired from commercial sources. All chemicals were of ACS reagent grade. Stock solutions of benzene and chlorobenzene were created by diluting them in double distilled water, resulting in a concentration of 1 × 10⁻² M for each solution. Working solutions with the desired final concentrations (5 × 10⁻⁴ M) were prepared through serial dilution from the stock solutions before conducting measurements. All chemicals and stock solutions were stored in amber glass bottles and kept in a dark environment at room temperature to avoid degradation. The solutions were examined for potential contamination through absorption scans.

2.2. Methods

2.2.1. Ultraviolet Spectrometer

To conduct absorbance measurements, we utilize a Shimadzu UV-2450 spectrophotometer (Shimadzu, Kyoto, Japan). This device scans wavelengths ranging from 190 nm to 1100 nm, featuring a wavelength accuracy of 0.3 nm and a resolution of 0.1 nm. The instrument is equipped with a single monochromator and a high-performance holographic grating. It utilizes two light sources: a 50 W halogen lamp and a deuterium lamp, which together provide a complete scanning range of 910 nm. The selected detector is the Photomultiplier 928. The spectrometer offers adjustable slit widths of 0.1, 0.2, 0.5, 1, and 5 nm. In terms of dimensions, the overall size of the spectrometer is 57 × 66 × 27.5 cm, while the sample compartment measures 15 × 26 × 12 cm. To ensure the maintenance of optical and electrical components as well as the sample compartment, a nitrogen gas inlet is installed at the rear of the instrument for continuous purging.

2.2.2. Cryostat and Cell

Figure 1 shows a schematic of the cell attached to the cold head of the cryostat. The cryostat was sourced from International Cryogenics Inc. USA (Indianapolis, IN, USA). The central dewar, which cools the cell, has a capacity of up to 12 L of liquid nitrogen. A copper block was utilized to connect the cell and the cold head, facilitating thermal contact and cooling of the cell. The entire cell was housed within a vacuum chamber that provides thermal insulation for both the cell and the cold head of the cryostat, protecting them from humidity condensation. The cryostat was equipped with four vacuum ports; one served as the connection from the cell to the sample system. Two of these ports were designated for measuring the vacuum pressure within the cryostat. The remaining port provided access to the electrical wires for temperature monitoring. Two thermocouples were linked to a temperature controller that gauged the temperatures of both the cold block and the cell. The heating resistor, responsible for adjusting the cell’s temperature, was situated on the cold block. The vacuum surrounding the cell, maintained at 10⁻⁶ Torr, was achieved through a combination of a mechanical and a diffusion pump.

Low-temperature UV absorption measurements were performed using sapphire optical windows of 2.5 cm diameter and 3 mm thickness. A high purity copper gasket (O-ring) of 2 cm outer diameter and 1 cm internal diameter was placed between the two windows as a spacer. The sample was loaded within the copper O-ring. The windows and spacer were housed together in an aluminum casing. The pathlength of the cell was 1 mm. A cell lid was then put on to fix everything inside the housing. The housing consisted of custom-made screw threading (¼″—28 drill tap size) for cold-stage attachment.

3. Results

3.1. Benzene

Our experimental results for the two transitions in the UV are shown in Figure 2. The concentration of benzene in water is 5 × 10⁻⁴ M. The pathlength of the cell is 1 mm.

The spectra are shown for temperatures at 292 K (liquid) and from 273 K to 78 K (solid solution). The high energy (Figure 2a) transition ¹B_1u $\leftarrow$ ¹A_1g is shown between 190 nm and 210 nm. The low energy (Figure 2b) transition ¹B_2u $\leftarrow$ ¹A_1g is shown between 220 and 270 nm. The high energy transition has a peak wavelength of 204 nm at 292 K. The liquid water spectrum shows a band feature with high overlap of the vibrational transitions. Upon lowering the temperature, a general trend of increase in relative intensity and band narrowing can be observed. This observation is plausible since benzene molecules isolated in water ice at cryogenic temperatures yield sharp vibronic bands. The band maximum shifts from 204.5 nm to 203.19 nm, showing a slight blue shift.

The low-energy electronic transition, ¹B_2u $\leftarrow$ ¹A_1g appears significantly weaker than the high-energy transition. The peak maximum for this transition is seen at 259.83 nm. These vibronic bands are blue shifted as the temperature decreases.

The temperature dependence of the integrated bands is shown in Figure 3, which shows a plot of the integrated band versus the absolute temperature. The band ¹B_1u $\leftarrow$ ¹A_1g is integrated between 190 and 220 nm. The band ¹B_2u $\leftarrow$ ¹A_1g is integrated between 225 and 270 nm. The overall ¹B_1u $\leftarrow$ ¹A_1g band intensity increases when the temperature decreases until 200 K. It decreases slightly at 170 K and begins to increase again reaching a maximum at 140 K. Below 140 K there is a gradual decrease until we reach 78 K. The ¹B_2u $\leftarrow$ ¹A_1g band shows almost constant integrated absorbance with a gradual decrease when the temperature decreases.

Table 1. Benzene in water ice absorption at 78 K. Peak position in wavelength (λ, nm) and wavenumber (σ, cm⁻¹), difference (Diff.) with respect to the origin, tentative assignment (assgs.), and spectroscopic (Spectrosc.) notation.

Benzene Transition 1$B_{1u}\leftarrow$1$A_{1g}$
λ nm	σ cm−1	Diff. cm−1	Tentative assgs. cm−1	Spectrosc. notation
192.25	52,016	4591	521 + 4 × 923 + 2 × 238	$6_0^1{1}_0^4{16}_0^2$
195.31	51,201	3776	521 + 3 × 923 + 2 × 238	$6_0^1{1}_0^3{16}_0^2$
199.75	50,063	2638	521 + 2 × 923 + 2 × 148	$6_0^1{1}_0^2{\left(latt\right)}_0^2$
203.00	49,261	1836	521 + 923 + 2 × 238	$6_0^1{1}_0^1{16}_0^2$
208.56	47,948	523	521	$6_0^1$
210.86	47,425	0	0	origin
Benzene Transition 1$B_{2u}\leftarrow$1$A_{1g}$
λ nm	σ cm−1	Diff. cm−1	Tentative assgs. cm−1	Spectrosc. notation
226.88	44,076	6047	521 + 6 × 923	$6_0^1{1}_0^6$
230.01	43,477	5448	6 × 923	$1_0^6$
232.76	42,963	4934	5 × 923 + 2 × 238	$1_0^5{16}_0^2$
236.32	42,315	4286	521 + 4 × 923	$6_0^1{1}_0^4$
238.76	41,883	3854	4 × 923	$1_0^4$
241.76	41,363	3334	521 + 3 × 923	$6_0^1{1}_0^3$
244.51	40,897	2868	3 × 923	$1_0^3$
247.20	40,453	2424	521 + 2 × 923	$6_0^1{1}_0^2$
250.02	39,997	1968	2 × 923	$1_0^2$
253.14	39,503	1474	521 + 923	$6_0^1{1}_0^1$
256.14	39,041	1012	923	$1_0^1$
259.14	38,589	560	521	$6_0^1$
260.89	38,330	301	2 × 148 $\left(lattice\right)$	${\left(lattice\right)}_0^2$
262.96	38,029	0	0	origin
266.96	37,459	−570	−521	$6_1^0$

shows the observed peak positions in wavelength (λ nm) and wavenumber (cm⁻¹), the difference with respect to the origin (Diff.), the tentative assignments (assgs.), and the spectroscopic (Spectrosc.) notation for the transitions ¹B_1u $\leftarrow$ ¹A_1g and ¹B_2u $\leftarrow$ ¹A_1g. The assignments of the ¹B_2u $\leftarrow$ ¹A_1g. transition are possible because all vibrational levels of the first excited electronic state (¹B_2u) of benzene are known [16,17]. The same vibrational levels are assumed to apply for the second excited electronic state (¹B_1u) because only the experimental values of ${\nu }_6=510 {\mathrm{c}\mathrm{m}}^{-1}$ and ${\nu }_1=910 {\mathrm{c}\mathrm{m}}^{-1}$ are known for the (¹B_1u) state. The two values are presented in a theoretical calculation of vibrational levels of the (¹B_1u) state [8]. Tentative assignments are shown with the magnitude of the individual vibrational frequencies of the excited states. The spectroscopic notation follows the mode assignment numbering [15] and the vibronic transition notation. In agreement with previous results [2], the intensity of band ¹B_1u $\leftarrow$ ¹A_1g goes down to zero around 210 nm at 78 K and shows an overall blue shift when the temperature decreases. We assign the first peak (208.63 nm) to the $6_0^1$ vibronic transition. Higher energy peak absorptions are identified as transitions of the type $6_0^1{1}_0^n$ (n = 1–4) in combination with the transition ${16}_0^2$ except for (n = 2) combined with the first overtone of a lattice vibration.

If the origin of the ¹B_2u $\leftarrow$ ¹A_1g transition (Table 1) is selected at 262.96 nm, there is complete agreement with previous assignments [2] of the $6_0^1$ and the progression $6_0^1{1}_0^n$ (n = 1–4). The origin at 262.96 nm also produces new tentative assignments for our listed peaks that include the $6_1^0$ for the peak at 266.96 nm, the first overtone of the lattice vibration (2 × 180 cm⁻¹) at 260.89 nm, the $6_0^1{1}_0^6$ at 236.88 nm, and a progression of the symmetric ring stretch (1) of the type $1_0^{n }$ (n = 1–4, and 6). The peak at 232.76 nm was tentatively assigned to $1_0^5{16}_0^2$ but it could also be the combination $6_0^1{1}_0^5$.

3.2. Chlorobenzene

The absorption spectra of chlorobenzene in water–ice are shown in Figure 4a from 190 nm to 290 nm at temperatures ranging from 292 K to 78 K. Figure 4b shows an expanded view of the spectra between 225 nm and 285 nm.

The measurements were taken with the 1 mm pathlength cell and a solution of chlorobenzene in water of concentration 5 × 10⁻⁴ M. The spectra reveal two prominent bands: Figure 4a shows the strong ¹A₁ $\leftarrow$ ¹A₁ transition that appears in the region from 190 nm to 230 nm. The low-energy transition shown in Figure 4b appears in the range from 230 nm to 290 nm. It is less intense than the high-energy transition and corresponds to the ¹B₂ $\leftarrow$ ¹A₁ transition. At 292 K, the ¹A₁ $\leftarrow$ ¹A₁ transition (see Figure 4a) lacks distinct features, displaying a broad, sloping curve towards shorter wavelengths. However, as the temperature decreases, the matrix isolation of chlorobenzene molecules leads to well-defined band peaks. The bands become more prominent and achieve optimal resolution at 78 K, with a strong band feature appearing at 208 nm. This enhancement in spectral resolution can be attributed to the effect of low temperature.

The temperature dependence of the integrated bands is shown in Figure 5, which shows a plot of the integrated band versus the absolute temperature. The band corresponding to the ¹A₁ $\leftarrow$ ¹A₁ transition is integrated between 190 and 230 nm. The band corresponding to the ¹B₂ $\leftarrow$ ¹A₁ transition is integrated between 230 and 280 nm. The overall ¹A₁ $\leftarrow$ ¹A₁ band intensity decreases when the temperature decreases slowly until 200 K, and then more rapidly between 200 K and 140 K. Below 140 K there is a gradual decrease until we reach 78 K. The main change occurs between 200 K and 140 K. The integrated ¹B₂ $\leftarrow$ ¹A₁ band decreases slowly but, similar to benzene, does not seem to be affected by the temperature change.

Table 2. Chlorobenzene in water–ice absorption at 78K. Peak position in wavelength (λ, nm) and wavenumber (σ, cm⁻¹), difference (Diff.) with respect to the origin, tentative assignment (assgs.), and spectroscopic (Spectrosc.) notation.

λ nm	σ cm−1	Diff. cm−1	Tentative assgs. cm−1	Spectrosc. notation
Chlorobenzene Transition 1A1 $\leftarrow$ 1A1
192.85	51,854	6880	378 + 7 × 931	${{6a}_0^{1}1}_0^7$
196.25	50,955	5981	378 + 6 × 931	${{6a}_0^{1}1}_0^6$
200.10	49,975	5001	378 + 5 × 931	${{6a}_0^{1}1}_0^5$
203.70	49,092	4118	378 + 4 × 931	${{6a}_0^{1}1}_0^4$
207.80	48,123	3149	378 + 3 × 931	${{6a}_0^{1}1}_0^3$
212.90	46,970	1996	378 + 2 × 931	${{6a}_0^{1}1}_0^2$
215.75	46,350	1376	378 + 931	${{6a}_0^{1}1}_0^1$
219.55	45,548	574	2 × 287	${18b}_0^2$
222.35	44,974	0	0	origin
Chlorobenzene Transition 1B2 $\leftarrow$ 1A1
λ nm	σ cm−1	Diff. cm−1	Tentative assgs. cm−1	Spectrosc. notation
233.40	42,845	6848	378 + 7 × 931	${{6a}_0^{1}1}_0^7$
236.35	42,310	6313	2 × 378 + 6 × 931	${{6a}_0^{2}1}_0^6$
239.95	41,675	5678	1065 + 5 × 931	${{7a}_0^{1}1}_0^5$
242.20	41,288	5291	1564 + 4 × 931	${{8a}_0^{1}1}_0^4$
252.90	39,541	3544	2 × 378 + 3 × 931	${{6a}_0^{2}1}_0^3$
257.15	38,888	2891	1065 + 2 × 931	${{7a}_0^{1}1}_0^2$
259.75	38,499	2502	671 + 2 × 931	${{12}_0^{1}1}_0^2$
263.05	38,016	2019	1065 + 931	${{7a}_0^{1}1}_0^1$
266.30	37,552	1555	521 + 1065	${6b}_0^1{7a}_0^1$
270.80	36,928	931	931	$1_0^1$
273.21	36,603	606	521	${6b}_0^1$
277.82	35,997	0	0	origin

shows the observed peak positions in wavelength (nm) and wavenumber (cm⁻¹) at 78 K, the difference with respect to the lowest energy that we can measure in absorption, the tentative assignments, and spectroscopic notation for the ¹A₁ $\leftarrow$ ¹A₁ and ¹B₂ $\leftarrow$ ¹A₁ transitions. The origin of the ¹B₂ $\leftarrow$ ¹A₁ transition in the gas phase is around 37048 cm⁻¹ [19,20]. The origin of our transitions is not known but the strong dipole–dipole interaction between chlorobenzene (1.73 D) and water (1.84 D) is expected to lower the potential energy curve with respect to the gas phase origin for both S₂ and S₁ levels. We use the lowest energy measured in each transition to achieve the progression. The assignments are possible because all vibrational levels of the first excited electronic state (¹B₂) of chlorobenzene are known. The same vibrational levels are assumed to apply for the second excited electronic state (¹A₁). The tentative assignments are shown with the magnitude of the individual vibrational frequencies of the excited states [13,16]. The spectroscopic notation for normal modes follows the mode assignment [14,16] and the vibronic transition notation. The analysis of the ¹A₁ $\leftarrow$ ¹A₁ transition begins with the assignment of the shoulder at (222.35 nm) as the origin. The first peak (219.55 nm) is assigned to the first overtone (${18b}_0^2$) of the non-symmetric vibronic transition (18b). It continues with the progression ${6a}_0^1{1}_0^n$ (n = 1–7) where (6a) is the symmetric Cl sensitive vibration and (1) is the symmetric ring vibration.

The origin of the ¹B₂ $\leftarrow$ ¹A₁ transition is assumed to be the lowest energy peak measured at 277.82 nm. The first measured vibration is the ring deformation (6b) at 521 cm⁻¹. The second is the symmetric ring vibration (1) at 931 cm⁻¹. The combination band (${6b}_0^1{7a}_0^1$) listed at 1555 cm⁻¹ could also be the C-C stretch mode (${8b}_0^1$) at 1564 cm⁻¹. Higher energy assignments involve the progression of the symmetric ring mode ($1_0^n$) n = 1–7 in combination with (${7a}_0^1$), the Cl-sensitive modes (${12}_0^1$), (${6a}_0^1$) and (${6a}_0^2),$ and the CC stretch (${8a}_0^1$).

4. Discussion

Our temperature dependence study (292 K to 78 K) of benzene/water ice was performed with one concentration (5.0 × 10⁻⁴ M) that corresponds to a composition around 4 × 10⁻⁵ g/g at 200 K. The most recent work on benzene shows three regions that are clearly shown in the study of benzene absorption bands in the vacuum UV and UV regions for gas phase, solid at 90 K, and 25 K [8] and for benzene in water ice with compositions: 1, 0.5, 0.1, and 0.01 solutions (benzene/water ice) at 24 K [2]. As was explained before, there is a strong transition from the ground state (¹A_1g) to the third singlet state (¹E_1u) denoted as the ¹E_1u $\leftarrow$ ¹A_1g transition. It is symmetry allowed and peaks around 180 nm. The bands that we show in our experiment are symmetry forbidden but appear due to vibronic coupling and intensity borrowing. One is from the ground state to the second singlet state (¹B_1u), denoted as ¹B_1u $\leftarrow$ ¹A_1g. It is at the base of the allowed band (¹E_1u $\leftarrow$ ¹A_1g) and borrows intensity from it. Another band (weakest of the three) is from the ground state to the first excited singlet (¹B_2u) and is denoted as ¹B_2u $\leftarrow$ ¹A_1g. The two bands in our study are like the published work at a higher composition and a lower temperature (24 K).

Other investigations refer only to the weakest band (¹B_2u $\leftarrow$ ¹A_1g) of vapor phase benzene that were performed in the near ultraviolet region [6,7]. The same absorption was shown for the solid solution of benzene in cyclohexane at 77 K at concentration ≤ 0.01 M [3], reporting detailed analysis of site symmetry, lifetimes, and relative intensities of two different crystal forms of benzene molecules occluded in cyclohexane lattice. Another study shows the (¹B_2u $\leftarrow$ ¹A_1g) band of benzene dissolved in various cryogenic liquids investigated intensity, position, and shape of transitions [4].

The chlorobenzene fundamental normal modes of the ground state are known following the measurement of a Fourier transform infrared (FT-IR) spectrum, which utilized synchrotron radiation [18]. As part of a study of Rydberg and ionic states of chlorobenzene, transitions of the valence states are presented showing the spectra and positions of the ¹B₂ and ¹A₁ states [11]. Some vibronic transitions of the ¹B₂ $\leftarrow$ ¹A₁ have been tentatively assigned up to 2000 cm⁻¹, approximately [14]. We present tentative assignments up to 7000 cm⁻¹. To our knowledge, there are no analyses of the transition ¹A₁ $\leftarrow$ ¹A₁. All the excited state frequencies and mode notations have been reported [14,16,28]. Older spectroscopic studies of chlorobenzene refer to Raman spectroscopy in the gas and liquid phases, providing interpretations of the stronger spectral lines in terms of the modes of vibration [19]. The ultraviolet spectra of chlorobenzene in carbon tetrachloride, chloroform, cyclohexane, n-hexane, ethanol, and water, were obtained in solutions with concentrations ranging between 0.005 and 0.0001 M. Qualitative agreement with the predicted solvent effect according to theory of Bayliss was shown [5]. Solid chlorobenzene spectra around 14 K were obtained and compared with the vapor spectra [29]. Additionally, the near ultraviolet absorption spectrum of gas phase chlorobenzene was observed in the range of pressures 0.1–720 Torr and analyzed finding several progressions involving various totally symmetrical vibrations. The observations resulted in identification of new “allowed” and “forbidden” bands when compared with the absorption spectrum of the solid at 14 K [30].

A comparison of peak shifts and integrated absorbances is shown in

Table 3. Observed band peak positions and peak shifts at 292 K (l) and 78 K (s) in water. Integrated absorbances of the bands at several temperatures (K).

		Peak Position		Shift	Integrated Absorbance
Compound	Band	292 K nm	78 K nm	Δλ nm	292 (K)	260 (K)	230 (K)	200 (K)	170 (K)	140 (K)	110 (K)	78 (K)
C6H6	1B2u $\leftarrow$ 1A1g	253.8	253.1	0.7	41.84	44.91	51.06	56.67	55.75	76.42	74.37	69.55
	1B1u$\leftarrow$1A1g	206.1	203.8	2.3	21.26	21.26	21.01	20.92	20.78	20.62	20.50	20.42
C6H5-Cl	1B2 $\leftarrow$ 1A1	263.4	266.3	-2.9	79.98	79.61	78.87	78.54	74.31	70.75	68.54	65.23
	1A1 $\leftarrow$ 1A1	199.7	203.7	-4.0	51.98	51.88	51.77	51.59	51.44	51.30	51.18	51.08

, where the peak maximum values at 292 K and at 78 K, and their shifts as well as integrated areas of the bands at all temperatures, are shown. The position of the maximum of each band is shifted to slightly higher energies for benzene and to lower energies for chlorobenzene at low temperatures.

The surrounding water ice molecules give extra stability to the chlorobenzene molecules lowering each potential energy curve but not as much as for the benzene molecules. The dipole–dipole interaction is responsible for the chlorobenzene–water interaction compared to the weak quadrupole–dipole interaction between benzene and water.

An analysis of plots of integrated area of absorbance versus temperature in water and water–ice solutions in Figure 3, the high energy ¹B_1u $\leftarrow$¹A_1g and the low energy ¹B_2u $\leftarrow$¹A_1g for benzene, and in Figure 5 the high energy ¹A₁ $\leftarrow$ ¹A₁ and the low energy ¹B₂ $\leftarrow$ ¹A₁ for chlorobenzene with actual values are shown in Table 3. The integrated areas of the benzene–water ice bands ¹B_1u $\leftarrow$ ¹A_1g show an increase from 292 K to 200 K. This could be explained using the particle in a box argument. Due to resonance, the ¹B_1u $\leftarrow$ ¹A_1g band borrows intensity from the allowed ¹E_1u $\leftarrow$ ¹A_1g ring band. By decreasing the temperature the solvent cage volume around benzene decreases, changing the energy levels in a way that decreases the difference in energy between the two states (narrowing the resonance) and borrowing more intensity. Below 200 K the smaller volume of the cage continues, this time separating the levels more gradually, reducing the resonance and the intensity borrowing.

There is a strong band for chlorobenzene equivalent to the ¹E_1u $\leftarrow$ ¹A_1g of benzene that has been observed [11,12]. Its assignment has been given [11] as the combination band of ¹A₁ + ¹B₂. The resonance between the levels involved in the combination band ¹A₁ + ¹B₂ and the A₁ $\leftarrow$ ¹A₁ band is optimum at 292 K but the decrease in the solvent cage volume in water–ice at lower temperatures, separates more of the energy levels of the chlorobenzene equivalent to the ring (¹E_1u) level of benzene and the ¹A₁ level of chlorobenzene. The A₁ $\leftarrow$ ¹A₁ band of chlorobenzene borrows less intensity from the ¹A₁ + ¹B₂ combination band. Table 3 also shows that the integrated intensity of chlorobenzene is larger than the integrated intensity of benzene above 170 K. Below that temperature, the trend is reversed.

There is very little change in the integrated intensity of the ¹B_2u $\leftarrow$ ¹A_1g band of benzene and the ¹B₂ $\leftarrow$ ¹A₁ band of chlorobenzene. They both decrease slightly in intensity when the temperature decreases, and the chlorobenzene integrated intensity is always around 2.5 times larger than the integrated intensity of benzene between 292 K and 78 K. The larger integrated intensity of the chlorobenzene band could be the sharpening of the band due to the dipole–dipole interaction of chlorobenzene/water compared to the quadrupole–dipole interaction of benzene/water.

The polar solvent interactions of liquid water at 292 K causes strong solvation shell. Rapid fluctuations in the solvent shell (solvation dynamics) and solute-solvent interactions (like H-bonding) affect the vibrational modes responsible for the vibronic intensity, heavily broadening the ¹B_1u $\leftarrow$ ¹A_1g band of benzene and the ¹A₁ $\leftarrow$ ¹A₁ of chlorobenzene, essentially eliminating the fine structure and preventing clear resolution, making it appear as a broad, featureless band. The ¹B_2u $\leftarrow$ ¹A_1g of benzene and the ¹B₂ $\leftarrow$ ¹A₁ of chlorobenzene, although also vibronically allowed, are less affected, indicating that the vibrational modes couple differently to the water solvation, leading to better resolution of its vibrational features.

The temperatures and choice of crystalline water ice for the determination of the spectra are based on what is presently known about ocean worlds. There is promising information about molecules detected around Enceladus. The molar mass of benzene studied in our research is 78 g/mol. The Cassini spacecraft orbiting around Enceladus obtained data from the Cosmic Dust Analyzer (CDA) and the Ion and Neutral Mass Spectrometer (INMS) aboard. The instruments detected abundant cationic forms of a benzene ring, phenyl (${C_6{H}_5)}^{+}$, mass 77) and other aromatic structures [31]. These substances are in the ice grains that constitute the plumes of water erupting from the subsurface ocean of Enceladus rather than from surface contamination or other external sources. The existence of these complex molecules, in conjunction with previous discoveries of hydrothermal silica nanoparticles, strongly indicates the presence of active hydrothermal vents on the ocean floor. These vents facilitate the mixing of materials from the rocky core of Enceladus with the ocean water.

With respect to the selection of crystalline ice versus amorphous ice for our study, the known laboratory work indicates that water ice naturally forms a crystalline structure (hexagonal ice) below 273 K. However, amorphous ice forms when water vapor deposits on surfaces below about 130 K or when crystalline ice is bombarded with energetic protons at low temperatures causing it to lose its organized structure and become amorphous [32]. The same bombardment that produces amorphous surface ice is also responsible for the destruction of molecules that could be mixed with the ice. The molecules experience a brief lifespan because of intense particle bombardment, which gradually deteriorates them [33]. If intact molecules are going to be found, crystalline ice has the highest probability of encapsulating those molecules.

On earth, chlorobenzene is produced in industry that can find their way into the atmosphere and then precipitate in the North pole and the areas of Earth surrounding the North Pole. Chlorobenzene is a precursor of the polychlorinated biphenyls (PCB) [34]. The chlorine atom has lone pairs of electrons that can be delocalized into the ring via resonance (+R effect) [35]. However, the chlorine atom is also highly electronegative and withdraws electron density through its σ bonds (inductive effect, −I). Photolysis of chlorobenzene at >254 nm in liquid water produces almost exclusively phenol derivatives showing an inductive effect [24]. Biphenyl and terphenyl as well as their chlorinated isomers are formed in water ice [23] favoring a resonant effect. The behavior and fate of semi-volatile organic compounds in snow have been reported, summarizing PCB concentrations in the Canadian Arctic, the Agassiz (Canada) ice cap, the Amituk lake (Canada), and the European alps [27]. Our spectroscopic study of chlorobenzene in ice adds information about the levels involved in the absorption of UV radiation.

5. Conclusions

In this paper, the absorption spectra of the aromatic molecules benzene and chlorobenzene were obtained in transparent hexagonal water–ice solutions at temperatures between 273 K and 78 K. The molecules were studied at the same initial concentration (5 × 10⁻⁴ M) in water ice. In general, the larger integrated intensities and frequency shifts in the chlorobenzene bands compared to benzene, could be due to the dipole–dipole interaction of chlorobenzene/water compared to the quadrupole–dipole interaction of benzene/water.

The temperature range of our study covers some of the average temperatures that have been found on the icy moon of Saturn (Enceladus) and the northernmost part of Earth surrounding the North Pole. In Enceladus, intact aromatic molecules are found in crystalline ice. The radiation bombardment that produces amorphous surface ice is also responsible for the destruction of molecules that could be mixed with the ice. PCBs in the arctic zones of Earth could be produced by irradiation of chlorobenzene encapsulated in ice crystals.