Gas-Phase Infrared Action Spectroscopy of CH₂Cl⁺ and CH₃ClH⁺: Likely Protagonists in Chlorine Astrochemistry

Abstract

Two fundamental halocarbon ions, CH₂Cl⁺ and CH₃ClH⁺, were studied in the gas phase using the FELion 22-pole ion trap apparatus and the Free Electron Laser for Infrared eXperiments (FELIX) at Radboud University, Nijmegen (the Netherlands). The vibrational bands of a total of four isotopologs, CH₂^35,37Cl⁺ and CH₃^35,37ClH⁺, were observed in selected wavenumber regions between 500 and 2900 cm⁻¹ and then spectroscopically assigned based on the results of anharmonic force field calculations performed at the CCSD(T) level of theory. As the infrared photodissociation spectroscopy scheme employed probes singly Ne-tagged weakly bound complexes, complementary quantum-chemical calculations of selected species were also performed. The impact of tagging on the vibrational spectra of CH₂Cl⁺ and CH₃ClH⁺ is found to be virtually negligible for most bands; for CH₃ClH⁺–Ne, the observations suggest a proton-bound structural arrangement. The experimental band positions as well as the best estimate rotational molecular parameters given in this work provide a solid basis for future spectroscopic studies at high spectral resolutions.

1. Introduction

Out of the more than 300 inter- and circumstellar molecules detected to date, more than 25% contain heavy elements from the second row of the periodic table. With nearly fifty species, molecules containing silicon and sulfur are particularly abundant (an up-to-date list of astronomically detected molecules with complementary information is maintained at the Cologne Database for Molecular Spectroscopy (CDMS) [1,2] accessible online at https://cdms.astro.uni-koeln.de/ (accessed on 12 December 2023)).

In contrast, the number of chlorine-bearing molecules is much smaller. Only seven such species have been observed so far: HCl [3]; AlCl, NaCl, and KCl [4]; HCl⁺ [5]; H₂Cl⁺ [6]; and CH₃Cl [7]. The latter, pentatomic methyl chloride, or CH₃Cl, is the most complex chlorine-bearing species presently known in space and, thus, an interesting target for studies of chlorine astrochemistry. In this context, it is assumed that CH₃Cl may be formed through reactions on grain surfaces but also possibly in the gas phase through ion–molecule reactions, for example, in a scenario suggested by Garrod (cf., Ref. [8]).

$${{\mathrm{CH}}_3}^{+} + \mathrm{HCl} \to {\mathrm{CH}}_2{\mathrm{Cl}}^{+} + {\mathrm{H}}_2$$

(1)

$${\mathrm{CH}}_2{\mathrm{Cl}}^{+} + {\mathrm{H}}_2 \to {\mathrm{CH}}_3{\mathrm{ClH}}^{+} + h\nu$$

(2)

$${\mathrm{CH}}_3{\mathrm{ClH}}^{+} + e^{-} \to {\mathrm{CH}}_3{\mathrm{Cl}}^{+} + \mathrm{H}$$

(3)

Here, in the first step, the chloromethyl ion CH₂Cl⁺ is formed from a reaction of the methyl cation CH₃⁺ and HCl; in the second, the halonium ion CH₃ClH⁺ is produced via radiative association between CH₂Cl⁺ and molecular hydrogen, finally yielding CH₃Cl through dissociate recombination with a free electron.

While the CH₃⁺, HCl, H₂, and CH₃Cl species are all known to be present in astronomical environments (the non-polar CH₃⁺ molecular ion being detected only very recently through observations with the James Webb Space Telescope [9]), our knowledge of the CH₂Cl⁺ and CH₃ClH⁺ molecular ions also participating in the above chain of reactions is still highly fragmentary. From the viewpoint of laboratory experiments, only little spectroscopic information has been collected previously for the chloromethyl cation. Photoelectron spectroscopy of the CH₂Cl radical has provided a vibrational wavenumber $\nu =1040\left(30\right)$ cm⁻¹ of the Cl–C-stretching mode of CH₂Cl⁺ [10,11]. A somewhat more comprehensive picture of the vibrational spectrum has been obtained through infrared spectroscopy of CH₂Cl⁺ trapped in solid Ar matrices [12]. In these studies, four out of the six vibrational fundamentals were observed for the most abundant isotopic species, CH₂³⁵Cl⁺. Using precursors enriched in ¹³C or deuterium permitted the observation of four vibrational fundamentals of ¹³CH₂³⁵Cl⁺ or five of CD₂³⁵Cl⁺, respectively. Owing to a significant isotopic shift, the C–³⁷Cl stretching mode could also be observed for all three isotopic species. Direct infrared spectroscopic studies of CH₂Cl⁺ in the gas phase have not been reported yet. Hexatomic protonated methyl chloride, CH₃ClH⁺, has been known from mass spectrometry and studies of reaction kinetics [13,14]; however, it has never been spectroscopically studied.

In the present work, the vibrational spectra of the ³⁵Cl and ³⁷Cl isotopic species of CH₂Cl⁺ and CH₃ClH⁺ were observed in the gas phase, marking the first direct gas-phase infrared study of CH₂Cl⁺ and the first ever spectroscopic detection of CH₃ClH⁺. The characterization of both ions was accomplished in an ion trap using a messenger action spectroscopy scheme, i.e., employing infrared photodissociation (IRPD) of their weakly bound complexes with neon, CH₂Cl⁺-Ne, and CH₃ClH⁺-Ne. Spectroscopic analysis was supported based on complementary high-level quantum-chemical calculations performed at the CCSD(T) level of theory. A detailed account of the experimental and theoretical work as well as of the analysis will be given in the following.

2. Results and Discussion

2.1. Structures of CH₂Cl⁺, CH₃ClH⁺, and Their Weakly Bound Complexes with Ne

Equilibrium molecular structures of CH₂Cl⁺ and CH₃ClH⁺ (and their isoelectronic sulfur-bearing counterparts, cf. Section 4) were calculated at the CCSD(T) level of theory using different basis sets. Full sets of internal coordinates are given in Appendix A and Appendix B. Best-estimate equilibrium structures were calculated using the cc-pwCVQZ basis set and all electrons in the correlation treatment, providing a level of theory previously shown to be of high accuracy [15]. The fc-CCSD(T)/cc-pV(T + d)Z level was employed for the calculation of harmonic and anharmonic force fields of the bare ions and used for the interpretation of their vibrational spectra. The impact of Ne-tagging on the molecular structures and harmonic force fields was estimated at the fc-CCSD(T)/aug-cc-pV(T + d)Z level of theory. To determine the energetically favorable locations of the Ne atom with respect to the CH₂Cl⁺ and CH₃ClH⁺ ions, potential energy surfaces (PESs) were calculated using grids of appropriate size and sampling (see also Ref. [16]). Briefly, in these calculations, the fc-CCSD(T)/aug-cc-pV(T + d)Z equilibrium structures of CH₂Cl⁺ and CH₃ClH⁺ were kept fixed while varying the position of the Ne atom on the grid over an area of a few tens of Ų and at a spacing of 0.25 Å. At each of the grid points, single-point energy calculations were performed. PESs were calculated for the two mirror planes of CH₂Cl⁺ and for the single mirror plane of CH₃ClH⁺. The resulting potential energy maps of CH₂Cl⁺ are shown in Figure 1.

Out of the four unique local minima seen, the shallow ones corresponding to linear C–Cl–Ne and Cl–C–Ne cluster variants were not considered further. The global minimum arrangements of each plot, i.e., one H-bound in-plane variant (CH₂Cl⁺ Ne-ip, Figure 1, left map) and the out-of-plane variant (CH₂Cl⁺ Ne-oop, right), were finally structurally refined in fully relaxed calculations. The corresponding structures are shown in Figure 2, and the full sets of structural parameters are given in Appendix A. As can be seen, the molecular structure of CH₂Cl⁺ hardly changes upon Ne-tagging at either position, and as a consequence, also the harmonic vibrational wavenumbers of CH₂Cl⁺ collected in

Table 1. Harmonic vibrational wavenumbers of CH₂³⁵Cl⁺ and CH₂³⁵Cl⁺–Ne calculated at the CCSD(T)/aug-cc-pV(T + d)Z level of theory (in cm⁻¹) ¹.

Mode	Bare Ion	Ne–ip	Ne–oop
${\omega }_1$ CH sym stretching	3102 ($a_1$, 18)	3106 ($a^{\prime }$, 17)	3105 ($a^{\prime }$, 16)
${\omega }_2$ CH2 scissoring	1478 ($a_1$, 0.3)	1476 ($a^{\prime }$, 1)	1478 ($a^{\prime }$, 0.2)
${\omega }_3$ Cl–C stretching	1059 ($a_1$, 80)	1059 ($a^{\prime }$, 78)	1058 ($a^{\prime }$, 82)
${\omega }_4$ oop bending	1137 ($b_1$, 22)	1138 ($a^{″}$, 21)	1136 ($a^{\prime }$, 20)
${\omega }_5$ CH asym stretching	3239 ($b_2$, 51)	3243 ($a^{\prime }$, 59)	3243 ($a^{″}$, 50)
${\omega }_6$ CH2 wagging	1047 ($b_2$, 0.02)	1048 ($a^{\prime }$, 0.3)	1047 ($a^{″}$, 0.003)
${\omega }_7$ 2,3	⋯	93 ($a^{\prime }$, 21)	131 ($a^{″}$, 1)
${\omega }_8$ 2,4	⋯	70 ($a^{″}$, 0.2)	89 ($a^{\prime }$, 22)
${\omega }_9$ 2,5	⋯	25 ($a^{\prime }$, 1)	34 ($a^{\prime }$, 4)

¹ Mode symmetries and infrared band intensities (km/mol) are given in parentheses. Mode indices of the Ne-tagged variants are borrowed from bare CH₂Cl⁺ for the sake of comparability. ² Extra-low-frequency vibrational modes introduced through Ne-tag, arbitrary mode index. ³ H–Ne stretching (Ne-ip species) and CH₂ twisting (Ne-oop species). ⁴ C–H–Ne stretching oop bending (Ne-ip species) and C–Ne stretching (Ne-oop species). ⁵ C–H–Ne ip bending (Ne-ip species) and Cl–C–Ne bending (Ne-oop species).

show very little dependence on tagging. The Ne-ip and Ne-oop species are calculated to be almost isoenergetic, with the latter configuration being slightly more stable by about 0.1 kcal/mol. Ne bond dissociation energies under the consideration of an estimate of zero-point vibrational effects amount to 0.6 kcal/mol (Ne-ip) and 0.7 kcal/mol (Ne-oop), respectively.

For CH₃ClH⁺, by analogy with other rare gas-tagged protonated species, one would expect the Ne atom to be bound to the proton in a near-linear Cl-H⋯Ne fashion (e.g., Refs. [17,18,19]). Indeed, from the potential energy map obtained for the molecular mirror plane (Figure 3), this arrangement is identified as a deep, putatively global minimum. Another minimum is identified on the opposite side of the molecule and a third shallow one where Ne is arranged in a near-linear Cl-C-Ne fashion. Fully relaxed structures of the first two configurations (“p-bound” and “CH₃/Cl-bound”) are shown in Figure 4. According to those calculations, the p-bound variant is more stable by 0.4 kcal/mol. The third minimum as well as potential arrangements where the Ne atom would be located outside of the symmetry plane of CH₃ClH⁺ have not been studied further. The results of harmonic force field calculations to study the influence of Ne-tagging are presented in

Table 2. Harmonic vibrational wavenumbers of CH₃³⁵ClH⁺ and two CH₃³⁵ClH⁺–Ne variants calculated at the CCSD(T)/aug-cc-pV(T + d)Z level of theory (in cm⁻¹) ¹.

Mode	Bare Ion	p-bound	CH3/Cl-bound
${\omega }_1$ C–H asym stretching	3220 ($a^{\prime }$, 18)	3220 ($a^{\prime }$, 17)	3224 ($a^{\prime }$, 20)
${\omega }_2$ C–H sym stretching	3087 ($a^{\prime }$, 5)	3087 ($a^{\prime }$, 5)	3089 ($a^{\prime }$, 5)
${\omega }_3$ Cl–H stretching	2823 ($a^{\prime }$, 176)	2811 ($a^{\prime }$, 307)	2824 ($a^{\prime }$, 174)
${\omega }_4$ CH3 asym bending	1472 ($a^{\prime }$, 14)	1472 ($a^{\prime }$, 14)	1471 ($a^{\prime }$, 13)
${\omega }_5$ CH3 umbrella	1386 ($a^{\prime }$, 11)	1386 ($a^{\prime }$, 11)	1385 ($a^{\prime }$, 10)
${\omega }_6$ CH3 ip rocking	1118 ($a^{\prime }$, 3)	1120 ($a^{\prime }$, 3)	1118 ($a^{\prime }$, 3)
${\omega }_7$ C-Cl-H bending	771 ($a^{\prime }$, 13)	784 ($a^{\prime }$, 11)	773 ($a^{\prime }$, 14)
${\omega }_8$ Cl–C stretching	565 ($a^{\prime }$, 51)	570 ($a^{\prime }$, 50)	566 ($a^{\prime }$, 50)
${\omega }_9$ C–H asym stretching	3238 ($a^{″}$, 15)	3237 ($a^{″}$, 15)	3238 ($a^{″}$, 15)
${\omega }_{10}$ CH3 asym bending	1468 ($a^{″}$, 14)	1469 ($a^{″}$, 14)	1470 ($a^{″}$, 12)
${\omega }_{11}$ CH3 oop rocking	1040 ($a^{″}$, 0.5)	1040 ($a^{″}$, 0.5)	1040 ($a^{″}$, 0.4)
${\omega }_{12}$ torsion	202 ($a^{″}$, 39)	244 ($a^{″}$, 43)	211 ($a^{″}$, 42)
${\omega }_{13}$ 2,3	⋯	90 ($a^{\prime }$, 7)	69 ($a^{\prime }$, 8)
${\omega }_{14}$ 2,4	⋯	72 ($a^{″}$, 7)	46 ($a^{\prime }$, 0.4)
${\omega }_{15}$ 2,5	⋯	20 ($a^{\prime }$, 7)	40 ($a^{″}$, 10)

¹ Mode symmetries and infrared band intensities (km/mol) are given in parentheses. ² Extra-low-frequency vibrational modes introduced through Ne-tag, arbitrary mode index. ³ H–Ne stretching. ⁴ Ne–H–Cl oop bending (p-bound species) and Ne–H–Cl ip bending (CH₃/Cl-bound species). ⁵ Ne–H–Cl ip bending (p-bound species) and Ne–H–Cl oop bending (CH₃/Cl-bound species).

. As can be seen, the influence of tagging is virtually negligible for the majority, but not all, of the vibrational fundamentals (see further discussion in Section 2.3). Under the consideration of zero-point vibrational effects, the Ne bond dissociation energies are 1.0  kcal/mol and 0.7  kcal/mol for the p-bound and CH₃/Cl-bound species, respectively.

2.2. IRPD Spectrum of CH₂³⁵Cl⁺–Ne

The $m/z=69$ filtered IRPD spectrum from 900 to 1600 cm⁻¹ is shown in Figure 5 (top red trace). From

Table 3. Harmonic and anharmonic fc-CCSD(T)/cc-pV(T + d)Z vibrational wavenumbers (in cm⁻¹) and infrared band intensities (int, km/mol), experimental wavenumbers (exp, cm⁻¹), full width at half maximum of observed bands (FWHM, cm⁻¹), and difference between observed and calculated anharmonic wavenumbers $\delta$ (cm⁻¹) of CH₂Cl⁺.

Mode	CH235Cl+
	Harm	Anharm	Int	Exp 1	FWHM	$\mathbf{\delta }$
${\nu }_1\left(a_1\right)$ CH sym stretching	3105.2	2993.3	14	$nc$	⋯	⋯
${\nu }_2\left(a_1\right)$ CH2 scissoring	1491.1	1451.1	0.7	⋯	⋯	⋯
${\nu }_3\left(a_1\right)$ Cl–C stretching	1061.8	1045.6	81	1048	13	2
${\nu }_4\left(b_1\right)$ oop bending	1151.9	1134.1	27	1129	17	−5
${\nu }_5\left(b_2\right)$ CH asym stretching	3243.9	3101.1	47	$nc$	⋯	⋯
${\nu }_6\left(b_2\right)$ CH2 wagging	1054.8	1038.2	<0.01	⋯	⋯	⋯
Mode	CH237Cl+
	Harm	Anharm	Int	Exp 1	FWHM	$\delta$
${\nu }_1\left(a_1\right)$ CH sym stretching	3105.2	2993.3	14	$nc$	⋯	⋯
${\nu }_2\left(a_1\right)$ CH2 scissoring	1491.0	1451.0	0.7	⋯	⋯	⋯
${\nu }_3\left(a_1\right)$ Cl–C stretching	1053.6	1037.6	82	1037	15	−1
${\nu }_4\left(b_1\right)$ oop bending	1151.8	1134.0	27	1125	13	−9
${\nu }_5\left(b_2\right)$ CH asym stretching	3243.9	3101.1	47	$nc$	⋯	⋯
${\nu }_6\left(b_2\right)$ CH2 wagging	1054.2	1037.6	<0.01	⋯	⋯	⋯

¹ Wavenumber errors amount to 0.5% of the band position; $nc$: not covered.

, it can be seen that four fundamental modes of the CH₂³⁵Cl⁺ species are calculated in this wavenumber range: the ${\nu }_6$ mode at 1038 cm⁻¹, ${\nu }_3$ at 1046 cm⁻¹, ${\nu }_4$ at 1134 cm⁻¹, and ${\nu }_2$ at 1451 cm⁻¹, only two of which have sizable calculated infrared intensities: ${\nu }_3$ (Cl–C stretching) and ${\nu }_4$ (out-of-plane (oop) bending). While infrared intensities are not strictly related to the intensities in the action spectroscopy scheme employed here, usually good qualitative agreement is observed. Clearly, ${\nu }_3$ and ${\nu }_4$ are detected at 1048 cm⁻¹ and 1129 cm⁻¹, respectively, which is in very good agreement with the anharmonic CCSD(T)/cc-pV(T + d)Z values (1046 and 1134 cm⁻¹, as shown in Table 3) and also show proper intensity behavior. Both ${\nu }_3$ and ${\nu }_4$ show weak satellite band progressions with approximate spacings of 20 cm⁻¹ to their blue sides (see inset in Figure 5), which is thought to originate from combination modes with the participation of the Ne-tag (most likely involving the ${\omega }_9$ fundamental (Table 1); see, e.g., Refs. [20,21]). The CH₂ scissoring mode ${\nu }_2$ is calculated to be very weak and would be located in a region affected by somewhat higher noise and some baseline instabilities. The CH₂ wagging mode ${\nu }_6$ is calculated to have almost no infrared intensity and is either too weak to be detected or overlapped with the much stronger ${\nu }_3$ band. Apart from that, no other clear spectroscopic feature is observed. As concluded from harmonic fc-CCSD(T)/aug-cc-pV(T + d)Z calculations of bare CH₂Cl⁺ and its singly tagged Ne-clusters (Table 1), tagging is expected to have a negligible influence on the IRPD spectrum of this ion.

2.3. IRPD Spectra of CH₂³⁷Cl⁺–Ne and CH₃³⁵ClH⁺–Ne

Because CH₂³⁷Cl⁺–Ne and CH₃³⁵ClH⁺–Ne are isobaric species ($m/z=71$), both ions were present in the ion trap, and hence, the corresponding spectrum comprises the spectra of both ions superimposed. The center trace in Figure 5 shows the final spectrum obtained in the wavenumber regions from 500 to 1530 cm⁻¹ and from 2625 to 2740 cm⁻¹. The two regions feature no less than seven vibrational bands, all but one of which can be assigned in a straightforward fashion based on the CCSD(T)/cc-pV(T + d)Z force field calculations. The two vibrational bands located at 1037 cm⁻¹ and 1125 cm⁻¹ are assigned as the strong ${\nu }_3$ and ${\nu }_4$ vibrational fundamentals of CH₂³⁷Cl⁺ (cf. Table 3). By comparison with the results from the CCSD(T)/cc-pV(T + d)Z force field calculations collected in

Table 4. Harmonic and anharmonic fc-CCSD(T)/cc-pV(T + d)Z vibrational wavenumbers (in cm⁻¹) and infrared band intensities (int, km/mol), experimental wavenumbers (exp, cm⁻¹), full width at half maximum of observed bands (FWHM, cm⁻¹), and difference between observed and calculated wavenumbers $\delta$ (cm⁻¹) of CH₃ClH⁺.

Mode	CH335ClH+
	Harm	Anharm	Int	Exp 1	FWHM	$\mathbf{\delta }$
${\nu }_1\left(a^{\prime }\right)$ C–H asym stretching	3225.4	3084.9	16	$nc$	⋯	⋯
${\nu }_2\left(a^{\prime }\right)$ C–H sym stretching	3091.6	2978.8	5	$nc$	⋯	⋯
${\nu }_3\left(a^{\prime }\right)$ Cl–H stretching	2829.5	2713.5	173	2677	15	−37
${\nu }_4\left(a^{\prime }\right)$ CH3 asym bending	1465.1	1420.6	15	1415	10	−6
${\nu }_5\left(a^{\prime }\right)$ CH3 umbrella	1378.2	1348.1	10	1345	8	−3
${\nu }_6\left(a^{\prime }\right)$ CH3 ip rocking	1113.6	1080.8	3	1083	9	2
${\nu }_7\left(a^{\prime }\right)$ C-Cl-H bending	768.4	757.1	14	766	9	9
${\nu }_8\left(a^{\prime }\right)$ Cl–C stretching	569.8	544.1	55	551	11	7
${\nu }_9\left(a^{″}\right)$ C–H asym stretching	3242.9	3100.6	14	$nc$	⋯	⋯
${\nu }_{10}\left(a^{″}\right)$ CH3 asym bending	1462.5	1419.6	15	1415	10	−5
${\nu }_{11}\left(a^{″}\right)$ CH3 oop rocking	1033.2	1020.6	1	blended	⋯	⋯
${\nu }_{12}\left(a^{″}\right)$ torsion	205.5	177.4	45	$nc$	⋯	⋯
Mode	CH337ClH+
	Harm	Anharm	Int	Exp 1	FWHM	$\delta$
${\nu }_1\left(a^{\prime }\right)$ C–H asym stretching	3225.4	3084.9	16	$nc$	⋯	⋯
${\nu }_2\left(a^{\prime }\right)$ C–H sym stretching	3091.6	2978.8	5	$nc$	⋯	⋯
${\nu }_3\left(a^{\prime }\right)$ Cl–H stretching	2827.3	2711.5	173	2678	20	−34
${\nu }_4\left(a^{\prime }\right)$ CH3 asym bending	1465.1	1420.5	15	1416	13	−5
${\nu }_5\left(a^{\prime }\right)$ CH3 umbrella	1377.9	1347.9	10	1344	11	−4
${\nu }_6\left(a^{\prime }\right)$ CH3 ip rocking	1112.7	1079.9	3	1082	11	2
${\nu }_7\left(a^{\prime }\right)$ C-Cl-H bending	768.1	756.8	14	765	9	8
${\nu }_8\left(a^{\prime }\right)$ Cl–C stretching	565.3	539.9	56	547	11	7
${\nu }_9\left(a^{″}\right)$ C–H asym stretching	3242.9	3100.6	14	$nc$	⋯	⋯
${\nu }_{10}\left(a^{″}\right)$ CH3 asym bending	1462.5	1419.6	15	1416	13	−4
${\nu }_{11}\left(a^{″}\right)$ CH3 oop rocking	1032.8	1020.2	1	1023	12	3
${\nu }_{12}\left(a^{″}\right)$ torsion	205.4	177.3	45	$nc$	⋯	⋯

¹ Wavenumber errors amount to 0.5% of the band position; $nc$: not covered.

, five bands are assigned as to originate from CH₃³⁵ClH⁺, marking the first spectroscopic detection of this molecular ion. CH₃ClH⁺ has a total of $3N-6=12$ vibrational fundamentals, four of which are lying outside of the spectral regions covered here, namely, the three C–H stretching modes ${\nu }_1$ (calculated at 3085 cm⁻¹), ${\nu }_2$ (2979 cm⁻¹), and ${\nu }_9$ (3101 cm⁻¹), as well as the low-energy torsional mode ${\nu }_{12}$ (177 cm⁻¹). Most other fundamentals are clearly identified, i.e., the Cl–C stretching mode ${\nu }_8$ at 551 cm⁻¹, the C–Cl–H bending mode ${\nu }_7$ at 766 cm⁻¹, the CH₃ in-plane rocking mode ${\nu }_6$ at 1083 cm⁻¹, and the CH₃ umbrella mode ${\nu }_5$ at 1345 cm⁻¹. According to the calculations, the strong feature at 1420 cm⁻¹ is from an overlap of the energetically nearly degenerate CH₃ asymmetric bending modes ${\nu }_4\left(a^{\prime }\right)$ and ${\nu }_{10}\left(a^{″}\right)$. The vibrational band with the largest calculated infrared intensity (Table 4) and indeed the strongest feature in the spectrum covered here is the Cl–H stretching mode ${\nu }_3$ detected at 2677 cm⁻¹. The weak out-of-plane rocking mode ${\nu }_{11}$ is overlapping with the much stronger ${\nu }_3$ band of CH₂³⁷Cl⁺. Overall, the agreement between the IRPD spectrum and the anharmonic CCSD(T)/cc-pV(T + d)Z force field calculation is excellent. The harmonic force field calculations of the bare ions and the singly tagged variants in Table 2 suggest that the influences of Ne should be subtle for most bands but noticeable for some. For the bands covered here, significant shifts might be expected for the ${\nu }_3$ (redshift), ${\nu }_7$ (blueshift), and, possibly, ${\nu }_8$ bands (blueshift) if Ne was connected to the site of protonation. Indeed, the experimental findings are qualitatively consistent with this geometrical arrangement, as the largest differences $\delta$ between experimental and calculated wavenumbers of all bands observed are found for these three modes that also agree with the predicted sign of the shift. Lastly, as seen from the inset in Figure 5 (blue center trace), similarly to the ${\nu }_3$ and ${\nu }_4$ bands in CH₂Cl⁺, the ${\nu }_8$ and ${\nu }_7$ bands (possibly also ${\nu }_3$) show weak satellite bands about 15 cm⁻¹ blueshifted from the fundamentals, presumably combination modes involving low-energy fundamentals introduced through Ne-tagging (most likely the ${\omega }_{15}$ mode, as shown in Table 2).

2.4. IRPD Spectrum of CH₃³⁷ClH⁺–Ne

The IRPD spectrum of the $m/z=73$ species covering the 500 to 1630 cm⁻¹ and 2505 to 2880 cm⁻¹ regions is shown in Figure 5 (green bottom trace). Clearly, the strong vibrational band quintett comprising the ${\nu }_8$, ${\nu }_7$, ${\nu }_5$, ${\nu }_{10}/{\nu }_4$, and ${\nu }_3$ bands nicely matches the pattern observed also for the CH₃³⁵ClH⁺–Ne isotopic species at $m/z=71$. Likewise, the ${\nu }_8$ and ${\nu }_7$ bands of CH₃³⁷ClH⁺–Ne show weaker yet clearly discernible shoulders to their blue sides. IRPD scans of the strong ${\nu }_8$ band reveal that depletion is nearly 100%, suggesting that the $m/z=73$ spectrum should provide a clean view of the CH₃³⁷ClH⁺–Ne species and not be affected by isobaric contamination. As a consequence, in addition to the weak CH₃ ip rocking mode ${\nu }_6$ band at 1082 cm⁻¹, now also the very weak CH₃ out-of-plane rocking mode ${\nu }_{11}$ can be identified at 1023 cm⁻¹. In addition to these vibrational fundamentals, the spectrum shows a spectroscopic feature at 1461 cm⁻¹ that is also seen as a somewhat less prominent signal in the $m/z=71$ spectrum (see bands marked with asterisks in Figure 5). From the anharmonic force field calculations, this feature is not compatible with a binary but, possibly, a ternary combination mode comprising the three energetically lowest fundamental modes, ${\nu }_7+{\nu }_8+{\nu }_{12}$. However, from the present investigation, a definitive assignment is not feasible.

3. Materials and Methods

3.1. Quantum-Chemical Calculations

Theoretical molecular structures of both CH₂Cl⁺ ($C_{2v}$) and CH₃ClH⁺ ($C_s$) have previously been provided on several occasions (e.g., Refs. [12,22,23,24,25]). In the present study, complementary quantum-chemical calculations of CH₂Cl⁺ and CH₃ClH⁺ (as well as their isostructural and isoelectronic sulfur analogs, H₂CS and CH₃SH) were performed at the coupled cluster single and double (CCSD) level augmented by a perturbative treatment of triple excitations (CCSD(T)) [26], together with correlation-consistent (augmented) polarized valence and (augmented) polarized weighted core–valence basis sets, i.e., cc-pVXZ [27], aug-cc-pVXZ [27,28,29], and cc-pwCVXZ [27,30] (with X = T, Q). For basis sets denoted as cc-pV(X + d)Z or aug-cc-pV(X + d)Z, an additional tight d function [31] was added to the chlorine (and sulfur) atom only, while for all other elements, cc-pV(X)Z or aug-cc-pV(X)Z was used, respectively. Equilibrium geometries were calculated using analytic gradient techniques [32], while harmonic frequencies were computed using analytic second-derivative techniques [33,34]. For anharmonic computations, second-order vibrational perturbation theory (VPT2) [35] was employed, and an additional numerical differentiation of analytic second derivatives was applied to obtain the third and fourth derivatives required for the application of VPT2 [34,36]. The frozen core approximation is indicated throughout with “fc”, and “ae” indicates that all electrons were used in the correlation treatment. All calculations were carried out using the CFOUR program package [37,38].

3.2. Experiment

Experimental infrared action spectroscopic characterization of CH₂Cl⁺ and CH₃ClH⁺ was performed in selected wavenumber ranges from 500 to 2900 cm⁻¹ using a cryogenic 22-pole ion trap apparatus FELion connected to the Free Electron Laser for Infrared eXperiments (FELIX) [39], located at Radboud University (Nijmegen, the Netherlands). The FELion apparatus and its central part, the 22-pole ion trap, have been described elsewhere in detail [40,41], and the experimental conditions applied were similar to those used in other recent studies of molecular ions [16,17,42,43,44]. CH₂^35,37Cl⁺ ($m/z=49,51$) and CH₃^35,37ClH⁺ ($m/z=51,53$) were produced in an ion source under the same experimental conditions from commercially available methyl chloride, CH₃Cl, using electron impact ionization at an electron energy of about 30 eV. Because of the high natural abundance of ³⁷Cl (24.24%), the corresponding heavy isotopologs CH₂³⁷Cl⁺ and CH₃³⁷ClH⁺ were easily accessible. It should be stressed that CH₂³⁷Cl⁺ and CH₃³⁵ClH⁺ are isobaric species ($m/z=51)$; hence, the corresponding spectra were expected to appear superimposed in the same scan.

After extracting the ion cloud from the source and selection in the first quadrupole mass analyzer, the $m/z$ of choice was introduced into the 22-pole ion trap, where it was cooled via a cold pulse of a 3:1 mixture of He:Ne kept at a nominal temperature of 8 K and at a high number density, leading to efficient tagging with Ne atoms. Per filling cycle of the ion trap, typically, several thousand cation–Ne complexes (CH₂^35,37Cl⁺–Ne at $m/z=69/71$ or CH₃^35,37ClH⁺–Ne at $m/z=71,73$) can be formed via three-body collisions (Figure A1 in Appendix C). After a selected storage time of typically 1.6 or 2.6 s, the trap content was extracted from the trap, mass-filtered in a second quadrupole mass analyzer, and counted using a very sensitive Daly-type detector.

In the infrared photodissociation (IRPD) action spectroscopy scheme employed herein, the number of singly tagged CH₂Cl⁺-Ne cluster ions (similar for CH₃ClH⁺-Ne) was monitored while the FELIX (FEL-2) IR radiation traversing the ion trap was tuned in wavenumber. FELIX was operated in a pulsed mode (10 Hz), with typical pulse energies of a few up to a few tens of mJ (measured at the exit of the ion trap) and a Fourier-limited full width at half maximum (FWHM) bandwidth on the order of 0.7%. When a vibrational mode of the cluster and the radiation source are coincident in wavenumber, dissociation of the cluster occurs, resulting in depletion in the ion–Ne counts. Finally, several power-normalized spectra were co-added to obtain the final spectrum. Details on the normalization procedure have been given in Ref. [41].

Since the weakly bound, rare gas Ne generally only slightly perturbs the structure of the ion, an IRPD spectrum is highly representative of that of the bare ion (e.g., Refs. [16,17,21,44]). Band parameters such as positions and FWHM were obtained by fitting (multi-component) Gaussian profiles to the experimental spectra. Typical wavenumber uncertainties of 0.5% are mainly dependent on calibration uncertainties due to the grating spectrum analyzer.

4. Conclusions

The present study reports on the first infrared gas-phase study of CH₂Cl⁺ and the first spectroscopic detection and characterization of CH₃ClH⁺, two fundamental chlorine-bearing ions possibly linked to the astrochemical gas-phase production of methyl chloride, CH₃Cl. Now that the IR spectra of CH₂Cl⁺ and CH₃ClH⁺ have been detected at low spectral resolution, corresponding studies at high resolution are imperative, in particular, to permit radio astronomical searches. Following the very recent technical developments in action spectroscopy of molecular ions, high-resolution infrared spectroscopy of both species may be performed using leak-out spectroscopy [45], a new method that does not require any rare gas tagging and has already been shown, on several occasions, to yield spectra of superb signal-to-noise ratios [46,47,48,49]. The method may even be combined with millimeter-wave radiation in a double-resonance fashion to derive accurate information about the pure rotational spectrum [46,47,50], a prerequisite for radio astronomy. Using the high-level CCSD(T) structural and force field calculations performed here, very good predictions of the ground state rotational and centrifugal distortion constants can be derived. These estimates may even be further improved using a simple scaling procedure, if experimental data of isostructural/isoelectronic species are available (see, e.g., Refs. [16,50,51]). As the Cl⁺ ion is isoelectronic with neutral atomic S, thioformaldehyde, H₂CS, and methyl mercaptan, CH₃SH, may potentially be used as calibrators, the pure rotational spectra of which are very well known from previous microwave and millimeter-wave studies (see, e.g., Refs. [52,53] for recent reports). Calculated and experimental rotational constants of H₂CS and CH₃SH (structural parameters are given in Appendix B) as well as calculated constants of CH₂Cl⁺ and CH₃ClH⁺ are collected in

Table 5. Rotational parameters of H₂CS, CH₂³⁵Cl⁺, CH₃SH, and CH₃³⁵ClH⁺ (in MHz) ¹.

Parameter	H2CS		CH235Cl+
	Calc.	Exp. [52]	Calc.	Scaled
$A_e$	295,043.772	⋯	278,232.178	⋯
$B_e$	17,770.906	⋯	17,929.887	⋯
$C_e$	16,761.346	⋯	16,844.397	⋯
$\mathsf{\Delta }{A}_0$	1928.402	⋯	2249.656	⋯
$\mathsf{\Delta }{B}_0$	76.366	⋯	91.765	⋯
$\mathsf{\Delta }{C}_0$	109.739	⋯	125.604	⋯
$A_0$	293,115.370	291,613.339	275,982.522	274,568.285
$B_0$	17,694.540	17,698.994	17,838.122	17,842.613
$C_0$	16,651.607	16,652.498	16,718.793	16,719.687
Parameter	CH3SH		CH335ClH+
	Calc.	Exp. [53] 2	Calc.	Scaled
$A_e$	103,863.953	⋯	103,144.040	⋯
$B_e$	13,029.847	⋯	12,472.858	⋯
$C_e$	12,493.723	⋯	12,016.948	⋯
$\mathsf{\Delta }{A}_0$	1054.186	⋯	1069.132	⋯
$\mathsf{\Delta }{B}_0$	131.546	⋯	207.375	⋯
$\mathsf{\Delta }{C}_0$	117.267	⋯	184.225	⋯
$A_0$	102,809.767	102,767.147	102,074.907	102,032.592
$B_0$	12,898.301	12,951.372	12,265.483	12,315.950
$C_0$	12,376.457	12,388.033	11,832.724	11,843.792

¹ Equilibrium rotational constants and zero-point vibrational contributions calculated at the ae-CCSD(T)/cc-pwCVQZ and fc-CCSD(T)/cc-pV(T + d)Z levels of theory, respectively. ² For the sake of comparability, the rotational constants from Ref. [53] given in the rho-axis system were converted to the principal axis system. Differences are virtually negligible.

. For the sulfur species, the agreement between the calculated and experimental ground state rotational constants is already very good. Consequently, the scaling factors, i.e., the ratios $A_{0,exp}$/$A_{0,calc}$, $B_{0,exp}$/$B_{0,calc}$, and $C_{0,exp}$/$C_{0,calc}$ are almost but not quite unity. Multiplying those factors with the calculated ground state rotational constants of CH₂Cl⁺ and CH₃ClH⁺ finally yields the scaled best-estimate rotational constants in the last column of Table 5. Both species are very polar with center-of-mass frame equilibrium dipole moment components ${\mu }_a=3.02$ D of CH₂Cl⁺ as well as ${\mu }_a=1.22$ D and ${\mu }_b=1.42$ D of CH₃ClH⁺.

From a chemical viewpoint, extending the spectroscopy of CH₂X⁺ and CH₃XH⁺ to halogens other than chlorine also seems appealing. The situation for X=Br, for example, appears similar to the one with X=Cl prior to the present study. To date, spectroscopic studies of CH₂Br⁺ have been restricted to infrared matrix isolation [12,54]. Protonated methyl bromide, CH₃BrH⁺, seems not to have been spectroscopically studied to date.