Article pubs.acs.org/JPCA
Rotational Spectrum, Conformational Composition, Intramolecular Hydrogen Bonding, and Quantum Chemical Calculations of Mercaptoacetonitrile (HSCH2CN), a Compound of Potential Astrochemical Interest Harald Møllendal,*,† Svein Samdal,† and Jean-Claude Guillemin‡ †
Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, NO-0315 Oslo, Norway ‡ Institut des Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11 Allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7, France S Supporting Information *
ABSTRACT: The microwave spectra of mercaptoacetonitrile (HSCH2CN) and one deuterated species (DSCH2CN) were investigated in the 7.5−124 GHz spectral interval. The spectra of two conformers denoted SC and AP were assigned. The H−S−C−C chain of atoms is synclinal in SC and anti-periplanar in AP. The ground state of SC is split into two substates separated by a comparatively small energy difference resulting in closely spaced transitions with equal intensities. Several transitions of the parent species of SC deviate from Watson’s Hamiltonian. Only slight improvements were obtained using a Hamiltonian that takes coupling between the two substates into account. Deviations from Watson’s Hamiltonian were also observed for the parent species of AP. However, the spectrum of the deuterated species, which was investigated only for the SC conformer, fits satisfactorily to Watson’s Hamiltonian. Relative intensity measurements found SC to be lower in energy than AP by 3.8(3) kJ/ mol. The strength of the intramolecular hydrogen bond between the thiol and cyano groups was estimated to be ∼2.1 kJ/mol. The microwave work was augmented by quantum chemical calculations at CCSD and MP2 levels using basis sets of minimum triple-ζ quality. Mercaptoacetonitrile has astrochemical interest, and the spectra presented herein should be useful for a potential identification of this compound in the interstellar medium. Three different ways of generating mercaptoacetonitrile from compounds already found in the interstellar medium were explored by quantum chemical calculations.
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INTRODUCTION Microwave (MW) spectroscopy has shown that the thiol group can form intramolecular hydrogen (H) bonds acting as a weak proton donor with a wide variety of acceptors. Examples include 2,2,2-trifluoroethanethiol (CF3CH2SH),1 trifluorothioacetic acid (CF3COSH),2 1,2-ethanedithiol (HSCH2CH2SH),3 thiiranemethanethiol (C2H3SCH 2SH),4 aminoethanethiol (H2NCH2CH2SH),5−7 methylthioglycolate (HSCH2COCH3),8 propa-1,2-dienethiol (H2CCCHSH),9 2-furanmethanethiol (C 4 H 3 OCH 2 SH), 1 0 cyclopropanemethanethiol (C 3 H 5 CH 2 SH), 1 1 propargyl mercaptan (HSCH 2 C CH),12−15 2-propenethiol (H2CCHCH2SH),16 3-mercaptopropionitrile (HSCH 2 CH 2 CN), 1 7 3-butene-1-thiol (HSCH2CH2CHCH2),18,19 3-butyne-1-thiol (HSCH2CH2CCH),20 and (Z)-3-mercapto-2-propenenitrile (HSCHCHCN).21 The present investigation of mercaptoacetonitrile (HSCH2CN) is an extension of our longstanding interest in internal H bonding in general and in thiols in particular.22 Intramolecular H bonding between a thiol group and a nitrile group is present in two of the compounds above, namely, 3mercaptopropionitrile (HSCH2CH2CN)17 and (Z)-3-mercapto-2-propenenitrile (HSCHCHCN).21 However, the geometrical orientation of the thiol and the nitrile groups in HSCH2CN is different from the corresponding orientations © XXXX American Chemical Society
in the H-bonded S−C−C−C syn clinal form o f HSCH2CH2CN20 and in the planar HSCHCHCN,21 and this was one reason to study the title compound. The genuine properties of weak internal H bonding can best be studied in the gas phase at low pressures because conformational and structural properties of compounds stabilized by this weak interaction are often perturbed in condensed phases. This makes MW spectroscopy, which is performed at low gas pressures and has extreme accuracy and resolution, the preferred method for such studies. Mercaptoacetonitrile may display rotational isomerism about the C−S bond resulting in the two conformers denoted SC and AP, which are depicted in Figure 1. Atom numbering is indicated on SC. The C2−C1−S4−H5 dihedral angle can conveniently be used to describe the conformational properties of HSCH2CN. This dihedral angle is synclinal in SC and anti-periplanar in AP. Intramolecular bonding between the thiol group and the nitrile group is possible only in SC. Determination of the energy difference between AP and SC could give a good indication of the strength of the H bond in SC, and this was one aim of the present study. Received: February 16, 2016 Revised: March 14, 2016
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potential generation of mercaptoacetonitrile in the gas phase from precursors present in the ISM.
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SYNTHESIS AND INSTRUMENTATION Synthesis. Mercaptoacetonitrile was synthesized as previously reported.43 The distillation to purify the crude product can easily be performed using a vacuum line (0.1 mbar) where small amounts of Amberlyst 15 are present both in the crude mixture and in the distilled compound. The heating was cautiously performed by immersion of the flask in a hot water bath (50−60 °C) and heating the glassware with a heat gun. The product was collected in a trap immersed in a −50 °C cold bath. A yield of distillation higher than 85% was obtained. The deuterated species DSCH2CN was produced by conditioning the MW cell with heavy water and then introducing the normal species. The concentration of DSCH2CN was roughly 30% of the total as judged by the intensities of the spectra of the normal and deuterated species. Spectroscopic Experiments. The vapor pressure of mercaptoacetonitrile, which is a liquid at room temperature, is roughly 110 Pa. Its MW spectrum was recorded at −30 °C or at room temperature in the 7.5−124 GHz spectral range at a pressure of a few pascal. The detailed description of the 50 kHz Stark-modulated spectrometer used in this investigation has been given elsewhere.44 Salient features of this instrument is its accuracy, which is 0.1 MHz for strong and isolated transitions, and its resolution of ∼0.50 MHz.
Figure 1. Models of the synclinal (SC) and anti-periplanar (AP) conformers of HSCH2CN. Atom numbering is indicated on the SC rotamer.
Astrochemistry is another interest of our two laboratories. Almost 200 different compounds (neutrals, radicals, cations, and anions) have so far been detected in the interstellar medium (ISM) or in circumstellar environments mainly by means of their rotational spectra.23 Many interstellar molecules contain oxygen, whereas sulfur of the same main group, is found in ∼15 compounds. One reason for the lower number of sulfur derivatives could be that sulfur compounds are generally more unstable kinetically, more reactive, and more difficult to synthesize than their oxygen analogues. Approximately 30 nitriles have been detected in the ISM.23 The majority of the nitriles contain carbon and hydrogen atoms in addition to the nitrile group. But there are also several interstellar compounds containing both heteroatoms as well as the nitrile group, for example, cyanic acid (HOCN),24 isothiocyanic acid (HSCN),25 cyanamide (H2NCN),26 cyanoformaldehyde (HC(O)CN), 2 7 E-cyanomethanimine (HNCHCN),28 and aminoacetonitrile (H2NCH2CN).29 Several metal cyanides, notably, sodium cyanide (NaCN),30 potassium cyanide (KCN),31 magnesium cyanide (MgCN),32 iron cyanide (FeCN),33 and cyanosilylidyne (SiCN),34 have also been detected in the ISM. It is thus well-established that compounds with a heterosubstituent connected to the nitrile group are indeed present in the ISM. Moreover, the CH2CN moiety is present in one of the interstellar molecules above, namely, aminoacetonitrile (H2NCH2CN).29 By analogy with aminoacetonitrile, the corresponding sulfur compound, (HSCH2CN), having the same CH2CN moiety, could exist in the ISM. Interestingly, the ISM harbors several potential precursors of mercaptoacetonitrile such as cyanomethylidyne (C2N),35 thioformaldehyde (H2CS),36 the cyano radical (CN),37 the cyanide cation (CN−),38 the cyanomethyl radical (CH2CN),39 hydrogen sulfide (H2S),40 the sulfanyl radical (SH),37 and sulfanylium (SH+).41 Lastly, vastly improved radio astronomy observatories such as the Atacama Large Millimeter/submillimeter Array (ALMA)42 have created a demand for rotational spectra of new interstellar candidate compounds such as mercaptoacetonitrile. The H bonding and conformational properties of mercaptoacetonitrile as well as its potential astrochemical relevance prompted the present study of its rotational spectrum. The experimental work is augmented with high-level quantum chemical calculations that predict a number of spectroscopic parameters, which are very helpful for the assignment of the rotational spectrum as well as interpretation of the experimental results. Theoretical calculations were also used to explore the
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RESULTS
Quantum Chemical Methods. The MP2 45 and CCSD46−48 methods were employed in this study. The MP2 calculations were undertaken using the Gaussian 09 program,49 whereas the CCSD computations were performed employing the Molpro program.50 Correlation-consistent wave functions of Dunning et al.51,52 were used. The calculations were performed employing the frozen-core approximation observing the default convergence criteria of the two programs. Barrier and Structure. The MP2/cc-pVQZ potential energy of rotation about the C1−S4 bond (Figure 2) was calculated by stepping the C2−C1−S4−H5 dihedral angle in 10° intervals and optimizing all geometrical parameters but this dihedral angle. The resulting function has three minima corresponding to the two mirror-image forms of SC and the AP conformer. MP2/cc-pVQZ calculations of optimized geometries, harmonic and anharmonic vibrational frequencies, harmonic zero-point vibrational energies, vibration−rotation (α’s) constants, equilibrium (re) and ground-state (r0) rotational constants, nuclear quadrupole coupling constants of the 14N nucleus, dipole moments, and Watson’s quartic and sextic centrifugal distortion constants53 were then performed for SC and AP. These MP2 parameters are listed in Tables S1 (SC) and S2 (AP) of the Supporting Information. It is seen from these two tables that the MP2 C2−C1−S4− H5 dihedral angle is 60.3° in SC and exactly 180° in AP. It is noted (Table S2) that AP has no imaginary vibrational frequencies, which is an indication that this conformer has a symmetry plane (Cs symmetry) and therefore no potential hump at this conformation. The electronic energy difference between SC and AP calculated from the entries in Tables S1 and S2 is 4.22 kJ/mol, with the former rotamer as the lowerenergy form. The energy difference becomes 4.68 kJ/mol when harmonic zero-point vibrational energies are taken into B
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Table 1. CCSD/cc-pV5Z Structures, Rotational Constants, and Dipole Moments of the SC and AP Conformers of HSCH2CN conformer
SC
AP
bond length (pm) 146.1 181.5 108.7 180.6 115.1 133.6 angle (deg) C2−C1−S4 113.2 C2−C1−H6 109.2 C2−C1−H7 109.4 S4−C1−H6 106.0 S4−C1−H7 110.8 H6−C1−H7 108.1 C1−S4−H4 96.2 C1−C2−N3 179.0 dihedral angle (deg) C2−C1−S4−H5 61.3 H6−C1−S4−H5 −179.1 H7−C1−S4−H5 −62.1 rotational constants (MHz) A 23 386.4 B 3109.5 C 2831.2 dipole momenta (debye) μa 2.38 μb 1.99 μc 0.74 μtot 3.19 energy differencec (kJ/mol) 0.0 C1−C2 C1−S4 C1−H6 C1−H7 C2−N3 S4−H5
Figure 2. MP2/cc-pVQZ electronic energy potential function for rotation about the C1−S4 bond. The energies relative to the global minimum (SC conformer) are given on the ordinate, whereas the values of C2−C1−S4−H5 dihedral angles are listed on the abscissa. This function has minima at 60.3, 299.7 (−60.3°), and 180° The minima at 60.3 and −60.3° correspond to the two SC mirror-image forms, whereas the minimum at 180° corresponds to AP. The energy of AP is 4.22 kJ/mol higher than the energy of SC. Maxima at 0 and 360° are 5.90 kJ/mol higher in energy than the energy of SC. The maxima at 129.1 and 230.9° are 6.94 kJ/mol higher, only 2.72 kJ/mol above the energy of AP.
account. The potential function has maxima at 0, 129.1, 230.9 (−129.1), and 360°. The maximum at 129.1° was found in a separate calculation (Table S3). The barrier heights are comparatively low, 5.90 kJ/mol at 0° (360°) and 6.94 kJ/mol at 129.1° (230.9°) relative to the electronic energy of SC. The barrier at 129.1° (230.9°), which separates the SC and AP conformers, is only 2.72 kJ/mol higher than the energy of AP. CCSD/cc-pV5Z computations are very costly and were only used to calculate the optimized geometries of the two conformers as well as their dipole moments. Cs symmetry was assumed for AP in these calculations. The energy of a conformation with a slightly perturbed AP-geometry was also computed to see whether there is a hump at the exact Cs symmetry. It turned out that the conformation with the exact Cs symmetry (AP) is lower in energy excluding the existence of a hump, just as found in the MP2 calculations above. The CCSD structures, rotational constants, principal-axes dipole moment components, and energy difference are displayed in Table 1. Further results of the geometry optimizations are shown in Tables S1 (SC) and S2 (AP). The CCSD C2−C1−S4−H5 dihedral angle in SC is 61.2° compared to the MP2-value of 60.3°. SC was found to be 4.70 kJ/mol lower in energy than AP, compared to the MP2 electronic energy difference of 4.22 kJ/mol and the zero-point corrected energy of 4.68 kJ/mol. Model Reactions in the Interstellar Medium. Is it possible to generate mercaptoacetonitrile in a spontaneous reaction in the gas phase from compounds that exist in the ISM? We explored by means of quantum chemical calculations three reactions that might lead to the title compound. The first of these involves the reaction between the cyanomethyl radical (CH2CN), which has been found in several places in the Universe,39,54 and H2S, which is a prevalent interstellar
146.1 182.2 108.6 108.6 115.0 133.6 109.1 108.7 108.7 110.5 110.5 109.4 95.0 180.0 180.0 −60.6 60.6 23 234.3 3187.5 2853.0 2.96 3.01 0.0b 4.22 4.70
a
Principal inertial axis components and total dipole moment. One D = 3.33564 × 10−30 C m. bFor symmetry reasons. cRelative to SC.
compound discovered in 197240 and found in many locations.23 These two compounds might produce HSCH2CN and the hydrogen radical: CH 2CN + H 2S → HSCH 2CN + H
(1)
UMP2/cc-pVTZ calculations (details in Table S4 of the Supporting Information) found that the electronic energy difference corrected for harmonic zero-point vibrational energies, ΔEZPE, is +46.25 kJ/mol for this reaction. The fact that ΔEZPE is positive means that this reaction is not spontaneous thermodynamically (positive Gibbs energy) under typical temperatures in the ISM. However, the relatively small value of ΔEZPE (+46.25 kJ/mol) is an indication that this reaction might perhaps occur on the surface of particles in the ISM. In the second reaction, the cyanomethyl radical reacts with the SH radical37 (data in Table S4): CH 2CN + SH → HSCH 2CN
(2)
UMP2/cc-pVTZ calculations yielded ΔEZPE = −316.91 kJ/mol for this reaction. This comparatively large and negative value of ΔEZPE indicates that this reaction is spontaneous (negative Gibbs energy) and might take place in the gas phase. The compound could get rid of the excess energy by radiation or C
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The Journal of Physical Chemistry A Table 2. Spectroscopic Constantsa of the 0+- and 0−-States of the SC Conformer of HSCH2CN 0+-state A (MHz) B (MHz) C (MHz) ΔJ (kHz) ΔJK (kHz) ΔK (kHz) δJ (kHz) δK (kHz) ΦJKc (Hz) ϕJ (Hz) ΔW (MHz) μbc (MHz) Nd stde
coupling parameters
23 108.824(55) 3105.1262(37) 2820.4783(36) 1.7593(61) −46.634(41) 539.7(39) 0.36953(35) 6.549(55) −0.135(25) 0.001 663(82)
0−-state
theoryb
23 112.829(62) 3105.2701(37) 2820.4838(37) 1.7501(61) −46.695(32) 514.5(39) 0.37136(37) 6.792(56) −0.260(24) 0.002 018(92)
23 350.8 3093.7 2795.3 1.81 −47.5 547 0.378 6.69 −0.154 0.001 59
8044(36) 0.158(31) 439 0.382
Experimental spectroscopic constants obtained from an ASMIXX63 fit. Spectrum in Table S8 of the Supporting Information. bTheoretical r0rotational constants and MP2 centrifugal distortion constants; see text. cFurther sextic constants preset at zero; see text. dNumber of transitions. e Standard deviation of the fit. a
on the barrier heights separating the two mirror-image forms and is expected to be of the order of several gigahertz. For example, ΔW is only 6891.76 MHz (0.230 cm−1)13 in the synclinal rotamer of propargyl mercaptan (HSCH2CCH), which is isoelectronic with mercaptoacetonitrile. The neardegeneracy of the 0+ and 0− substates may lead to perturbation of the rotational spectrum, which was indeed observed for propargyl mercaptan.13 The a- and b-type transitions of SC are purely rotational transitions occurring within either the 0+ or the 0− state, whose rotational constants were expected to differ little. These transitions were therefore expected to appear as characteristic closely spaced pairs of lines, whereas the c-type transitions are vibration−rotation transitions occurring between the 0+- and 0−-vibrational states and are separated by much larger frequency intervals. There will also be considerable contributions to the spectral richness from spectra belonging to vibrationally excited states of SC because the lowest anharmonic vibrational frequencies are 168.9, 206.1, 352.8, and 482.9 cm−1 according to the MP2 calculations (Table S1 of the Supporting Information). These states will consequently have significant Boltzmann factors at the observation temperatures (−30 and 22 °C). AP, which was calculated above by the CCSD method to be 4.70 kJ/mol higher in energy than SC, could also contribute to the spectral density because of its large a- and b-dipole moment components (∼3 D; Table 1). Survey spectra revealed a dense MW spectrum in accord with the theoretical predictions. The differences between the MP2 re- and r0-rotational constants (Table S1) were added to the CCSD rotational constants to predict theoretical r0-rotational constants (last column Table 2). These constants were used together with the MP2 quartic centrifugal distortion constants to predict the spectrum of the SC conformer. The comparatively strong b-type Q-branch series J1,J−1 ← J0,J was searched for first and was found close to the calculated frequencies. Most members of this series displayed characteristic pairs of relatively strong lines protruding from a background of weaker transitions. The intensities of each of the two components of a pair are equal. The members of the lower-frequency and higher-frequency components of these two Q-branch series were fitted separately to Watson’s
collisions. Branching reactions resulting in other compounds might also occur, but this has not been considered further. A UMP2 calculation assuming antiparallel spins of the two radicals indicated that this reaction takes place without a barrier. In the first step of the third hypothetical route to mercaptoacetonitrile, thioformaldehyde (H2CS), known to exist in many interstellar environments,23,36,54−57 reacts with the cyanide anion (CN−), discovered recently,38 to form the SCH2CN− anion, whose existence in the ISM is still unknown: H 2C = S + CN− → SCH 2CN−
(3)
MP2/aug-cc-pVTZ calculations predict that this reaction takes place without a barrier and is spontaneous because ΔEZPE is −179.44 kJ/mol. Reaction with H+ could lead to mercaptoacetonitrile in the second step: SCH 2CN− + H+ → HSCH 2CN
H3+,
(4) 58
which is a constituent of the ISM, might have a similar
role: SCH 2CN− + H3+ → HSCH 2CN + H 2
(5)
Reactions 4 and 5 are both spontaneous because MP2/augcc-pVTZ calculations predict that ΔEZPE = −1401.47 kJ/mol for the Reaction 4, whereas ΔEZPE = −985.54 kJ/mol for Reaction 5. Details are found in Table S5. Excess energy of Reactions 3−5 could be lost by radiation or collisions. Branching reactions may perhaps also occur, just as for Reaction 2. Microwave Spectrum and Assignment of the Spectrum of the SC conformer. The theoretical calculations above indicate that the major part of the spectrum must be due to the lower-energy SC conformer, which has dipole moment components (Table 1) along the a- (2.38 D), b- (1.99 D), and c-axis (0.74 D). The a- and b-spectra will be much stronger than the c-spectrum, because intensities are proportional to the square of the dipole moment component. Each torsional state of SC will be split into two substates due to the existence of two mirror-image forms. The lowest substate is denoted 0+, and the second-lowest state is called 0−. The energy difference between these two states, ΔW, will depend D
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The Journal of Physical Chemistry A Hamiltonian in the A-reduction form53 using Sørensen’s program Rotfit.59 The standard deviations of these fits were each ∼0.4 MHz. The spectroscopic constants obtained from the J1,J−1 ← J0,J series were used to predict the transitions of the J2,J−2 ← J1,J−1 Q-branch lines. Transitions with J > 14 could be incorporated in the fit without increasing the standard deviation of the fit significantly. However, transitions with lower values of J appear to be perturbed by several megahertz and were taken out of these Watson fits. Instead, transitions belonging to the J3,J−3 ← J2,J−2 and J4,J−4 ← J3,J−3 bQ series were gradually incorporated without any significant changes in the standard deviations of the fits. Ultimately, we succeeded assigning Q-branch transitions with values of J as large as 56 (564,52 ← 563,53 transition) for what turned out to be the 0+-state (see below), whereas Jmax was 55 for the 0− state (554,51 ← 553,52 transition). The standard deviation of each of these two fits was ∼0.4 MHz, which is larger than the experimental uncertainty estimated to be ∼0.10 MHz. Intensities cannot be used to assign unambiguously each of these two bQ-branch series to either the 0+- or to the 0−-state because they have practically identical intensities. An alternative method had to be used for this purpose: The difference between the A- and C-rotational constants (A − C) and Ray’s asymmetry parameter,60 κ, are obtained from a fit of Q-branch lines. The value of A − C depends on the torsion about the C1−S4 bond. The C2−C1−S4−H5 dihedral angle is presumed to be a little larger in the 0−-excited state than in the 0+-ground state. The effect of increasing this dihedral angle on the rotational constants was modeled by changing the said dihedral angle of the CCSD structure (Table 1) by +2° keeping the rest of the structure unchanged. The A, B, and C rotational constant changed by +4.82, −2.13, and −0.045 MHz in this calculation. The spectrum having the smallest value of A − C was therefore assumed to belong to the 0+-state, and the largest value of A − C was assigned to the 0− state. The a-type R-branch transitions were assigned next by their spectral positions, Stark effects, and by the use of the radio frequency microwave double resonance (RFMWDR) method.61 Ray’s asymmetry parameter,60 κ, is −0.97. The aR lines will therefore form characteristic pileups separated by approximately the sum of the B and C rotational constants (B + C). A part of the pile-up region of the J = 20 ← 19 transition is displayed in Figure 3. The assignments of several a R transitions were confirmed by RFMWDR experiments.61 An example is shown in Figure 4. It can be seen from Figures 3 and 4 that the frequency of an a R line with given rotational quantum numbers belonging to the 0+ state is separated from the transition of the 0− state with the same quantum numbers by a small frequency interval (a few MHz at most). In fact, they coalesce into a single, unresolvable line (split by less than ∼0.5 MHz) in many cases, especially for low values of J. The close spectral proximity of the resolved pairs made it impossible to say in a straightforward way whether a resolved a R line belonged to the 0+ or to the 0− state. Two separate Watson fits using only resolved aR lines belonging either to the 0+ or to the 0− state were performed to settle this assignment question. Relatively large standard deviations (∼0.4 MHz) were found for each of these fits, just as in the case of the two fits of the bQ-branch transitions above. The values found for A − C of these two aR fits should of course be the same as those obtained
Figure 3. Portion of the microwave spectrum of SC in the region of the J = 20 ← 19 a-type transitions taken at a field strength of roughly 110 V/cm. Transitions belonging to the 0+- and 0−-states appear as doublets separated by a few megahertz. The 0+-state transitions have lower frequencies than the 0− transitions. The number above each pair indicated the value of the K−1 pseudo quantum number. Overlapping of transitions occur frequently in this rich spectrum. The K−1 = 5 and K−1 = 11 of the 0− state are overlapped. The K−1 = 8 transitions overlap with the K−1 = 6 and are seen as a shoulder on the latter two transitions. Most of the lines not marked presumably belong to vibrationally excited states of SC.
Figure 4. RFMWDR spectrum of the closely spaced 0+ and 0− pairs of the ground-state J = 164 ← 154 transitions of SC using a radio frequency of 8.23 MHz. The pair marked X is assumed to belong to J = 164 ← 154 transition of a vibrationally excited state.
from the fits of the bQ transitions above, and this was used to obtain unambiguous assignments of the two spectra. The correct set of aR lines were then fitted together with the appropriate bQ lines and used to predict the frequencies of the b-type R-branch transitions of both states, which were readily found and included in the fits. No transitions were found to be split by nuclear quadrupole interactions of the 14N nucleus. Finally, coalescing transitions were included in the spectrum of the 0+ state but not in the spectrum of the 0− state. All quartic and two sextic centrifugal distortion constants, ΦJK and ϕJ, were varied in the fits with the remaining sextic constant preset at zero. Inclusion of additional sextic constants in the fits E
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MHz. However, a c-type vibrational spectrum would be very weak at this frequency, due to the fact that μc is calculated to be only 0.74 D, and intensities are proportional both to the square of the dipole moment component and the square of the frequency. Intensities are enhanced by lowering the temperature, and the spectrum was therefore recorded in the region around 8 GHz at −30 °C, which was the lowest temperature that could be employed due to the volatility of mercaptoacetonitrile. The transitions shown in Table S8 should be helpful for a potential identification of mercaptoacetonitrile in the ISM. The spectroscopic constants of Table 2 should predict frequencies outside the spectral region investigated in this work (7.5−124 GHz) precisely for the majority of transitions apart from K−1 = 2 aR- and J2,J−2 ← J1,J−1 Q-branch lines with J < 14. The Watson spectroscopic constants of Tables S6 and S7 are presumably perhaps more convenient to use and should produce fairly accurate predictions of transitions other than the “problematic” ones mentioned above. Table 2 reveals that rotational constants of the 0+ and 0− states are very similar. The largest difference of ∼4 MHz is seen for the A-rotational constant. The quartic centrifugal distortion constants of the two states are also similar, deviating by less than 5%. Much larger differences are obtained for the two sextic constants, but these constants have much larger relative standard deviations than the rotational and quartic centrifugal distortion constants. Comparison with the theoretical constants in the last column of Table 2 shows that the experimental A-rotational constants of the 0+ and 0− states are ∼1.0% smaller than the theoretical counterpart, whereas the experimental B- and C-rotational constants are 0.37 and 0.89%, respectively, smaller than the theoretical values in the last column. Differences of this magnitude are expected due to the approximate nature of the theoretical calculations. The theoretical quartic centrifugal distortion constants are in satisfactory agreement (better than 8%) with their experimental counterparts, and the two experimental sextic constants are only in fair agreement with theory. Spectrum of the SC Conformer of DSCH2CN. The CCSD structure of SC (Table 1) was used to calculate the shifts of the rotational constants resulting from deuteration of the thiol group. The shifts were added to the experimental rotational constants of the 0+ species. These constants were used to predict the frequencies of aR transitions, which were found close to the predicted frequencies. It was not possible to resolve any of these aR lines into their 0+ and 0− components because they are split by a very small frequency interval for this isotopologue. A set of preliminary spectroscopic constants obtained in this manner were used to locate bQ- and bR-branch transitions. The 0+ and 0− components of the b-type transitions were resolved in many, but not all, cases. The coalescing a- and b-type lines were used together with b-type transitions both of the 0+ as well as of the 0− substate, to determine the spectroscopic constants displayed in Table 3. The corresponding spectra are shown in Tables S9 and S10 of the Supporting Information. The maximum value of J = 60 for the 0+ lines and Jmax = 57 for the 0− transitions. The standard deviations of the two fits are 0.217 (0+ state), and 0.205 MHz (0− state; Table 3), respectively. The fact that many transitions are not split into two components often results in broad lines, and this is assumed to be the main reason for the rather large standard deviations of the two fits.
was attempted, but they did not result in significant improvements. The results are listed in Tables S6 (0+ state) and S7 (0− state) of the Supporting Information. Coalescing transitions are marked in Table S6 with an asterisk. It is seen from these two tables that standard deviations of the fits are 0.53 MHz for the 0+ state and 0.41 MHz for the 0− state, much larger than the measurement uncertainty. The larger value of the standard deviation of the 0+-state spectrum (0.54 MHz) compared to the 0−-state has mostly to do with the inclusion of coalescing transitions in the least-squares fit. Inspection of Tables S6 and S7 reveals that deviations based on calculations using Watson’s Hamiltonian are generally small. Transitions involving high values of J fit better than lines with smaller values of this quantum number. However, several aR lines with K−1 = 2 deviate by up to 2.5 MHz from the calculated frequencies. It is also characteristic that several of these transitions are not split into resolved 0+- and 0−-components. Deviations, but to a lesser extent, are observed in the case of the b R transitions of the K−1 = 1 ← 0 category (Tables S6 and S7). The Watson Hamiltonian can cope satisfactorily with the vast majority of the transitions, but it is inadequate in dealing with several transitions, especially those involving J < 15. A more advanced Hamiltonian is needed to obtain a better fit. The effective two-level rotation-vibration Hamiltonian by Harris et al.62 as implemented by Nielsen in the computer program ASMIXX63 was therefore employed. This Hamiltonian includes Watson-type quartic and sextic centrifugal distortion constants and coupling terms of the μ- and L-types.62,63 A total of 439 0+and 0−-resolved transitions taken from Tables S6 and S7 and shown in Table S8 were fitted using ASMIXX. The rotational, the quartic centrifugal distortion constants, and the sextic constants ΦJK and ϕJ were fitted together with the μbc coupling term and the energy difference, ΔW, between the 0+ and 0− states. Inclusion of the μab and μca coupling terms in fits was attempted, but unrealistic results were obtained in these cases. The results are shown in Table 2. The standard deviation of the fit to this Hamiltonian is 0.38 MHz (Table 2), which is only slightly better than found in the two Watson fits (Tables S6 and S7). Attempts to include the J < 15 members of the J2,J−2 ← J1,J−1 Q-branch lines in Nielsen’s program were made, but large deviations (2−10 MHz) were obtained. We therefore took these transitions out of the fit and added these tentatively assigned transitions at the end of Table S8. It was again found that K−1 = 2 aR lines fit poorly using the coupled Hamiltonian. The deviations are larger for the lower-J members of these K−1 = 2 aR lines than for higher J values. The spectroscopic constants of this Hamiltonian predict not only frequencies that are off by a few megahertz for low members of the K−1 = 2 lines, but also predict too large frequency separation of the 0+ and 0− components for these transitions, which often coalesce. Obviously, there are higher-order interaction terms that even this coupled Hamiltonian does not take properly into account. The relatively small coupling constant μbc = 0.159(31) MHz reflects the absence of large coupling interactions between the 0+ and 0− states. Interestingly, the energy difference between the lower 0+ state and the upper 0− state are rather precisely predicted to be ΔW = 8044(36) MHz. Pure vibrational c-type transitions should appear near this frequency, similar to what was observed for HSCH2CCH (6891.76 MHz).13 The spectrum was recorded from 7500 MHz toward higher frequencies. No characteristic c-type vibrational pileup similar to that observed for HSCH2CCH13 was noted around 8000 F
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The Journal of Physical Chemistry A Table 3. Spectroscopic Constantsa of the SC Conformer of DSCH2CN A0 (MHz) B0 (MHz) C0 (MHz) ΔJ (kHz) ΔJK (kHz) ΔK (kHz) δJ (kHz) δK (kHz) ΦJK (Hz) ϕJb (Hz) Nc stdd
0+ state
0− state
21 202.958(17) 3059.3835(16) 2786.2092(16) 1.8269(26) −37.7839(97) 403.5(11) 0.392 23(17) 8.021(32) −0.0123(43) 0.002 091(43) 310 0.217
21 203.136(17) 3059.3939(15) 2786.2042(15) 1.8306(25) −37.7242(95) 403.6(11) 0.392 37(17) 8.149(22) 0.0700(55) 0.002 157(43) 290 0.205
Table 4. Spectroscopic Constantsa of the AP Conformer of HSCH2CN A0 (MHz) B0 (MHz) C0 (MHz) ΔJ (kHz) ΔJK (kHz) ΔK (kHz) δJ (kHz) δK (kHz) ΦJ (Hz) ΦKJc (Hz) 2Pccd (u Å2) N std
a
A-reduction Ir-representation.53 Uncertainties represent one standard deviation. Spectra in Tables S9 and S10 of the Supporting Information. b Further sextic centrifugal distortion constants preset at zero. c Number of transitions. dStandard deviation of the fit.
experiment
theoryb
23 028.11(15) 3182.010(12) 2842.935(12) 2.195(59) −11.51(69) 1690(13) 0.5098(17) 44.37(32) 0.182(89) 323(51) 3.0034(2) 126 0.549
23 117.9 3169.9 2836.8 2.00 −50.3 539 0.428 6.37 0.005 80 −0.871 3.139
a
A-reduction Ir-representation.53 Uncertainties represent one standard deviation. Spectrum in Table S11 of the Supporting Information. b Theoretical r0-rotational constants (see text) and MP2 centrifugal distortion constants. cFurther sextic centrifugal distortion constants preset at zero. dPcc = (Ia + Ib − Ic)/2, where Ia, Ib, and Ic are the principal moments of inertia.
Inspections of Tables S9 and S10 show that the problems associated with the inclusion of aR K−1 = 2 and several b-type transitions in the Watson fits encountered for the normal species (see above) no longer exist. The spectroscopic constants (Table 3) should be able to predict frequencies outside the regions studied here with very high degree of precision. The principal inertial coordinates of the H atom of the thiol group can be calculated from the rotational constants of the parent and the deuterated species using Kraitchman’s equations.64 The rotational constants of the 0+ states of HSCH2CN (Table 2) and of DSCH2CN (Table 3) yielded |a| = 115.244(7), |b| = 90.766(9), and |c| = 108.361(8) pm in this manner, where the uncertainties were calculated from the standard deviations of the rotational constants. The CCSD values of these coordinates are |a| = 117.38, |b| = 91.76, and |c| = 107.79 pm (Table S1). These theoretical coordinates are not in good agreement with the experimental substitution counterparts, but it is assumed that the large-amplitude vibrations associated with the thiol group are responsible for most of this discrepancy. Assignment of the Spectrum of AP. The theoretical r0spectroscopic constants of this conformer (Table 4), obtained as described above for SC, were used to predict the approximate frequencies of its aR spectrum. The RFMWDR method was used to search for selected aR transitions, which were found close to the predicted frequencies. An example is shown in Figure 5. The frequencies of additional aR-branch lines could now be predicted rather precisely, and they were soon identified by their Stark effects and positions in the spectrum. This spectrum is much weaker than the spectrum of SC because AP is a high-energy form. Fits using Watson’s Hamiltonian were made, but it turned out that there are perturbations in this spectrum too. It was only possible to fit satisfactorily aR transitions with K−1 ≤ 4. Inclusion in the fit of aR lines assumed to have values of K−1 > 4 resulted in very large standard deviation of the fits, and these candidates were therefore ultimately taken out of the fit. The frequencies of b-type lines were predicted using the preliminary spectroscopic constants obtained from the aR-type transitions. Members of the J1,J−1 ← J0,J and J2,J−2 ← J1,J−1 b-type Q-branch series were identified by their intensities, Stark effect,
Figure 5. RFMWDR spectrum of the J = 83 ← 73 pair of transitions of AP using a radio frequency of 7.62 MHz. The lower-frequency component is the 83,6 ← 73,5 transition, and the higher-frequency component is the 83,5 ← 73,4 transition.
and fits to the Watson Hamiltonian. No definite assignments could be made for transitions belonging to bQ-branch series with K−1 > 2. It is assumed that this is also due to perturbations. Finally, several b-type R-branch lines were assigned, but the values of K−1 had to be restricted to 0 or 1 for similar reasons as above. The spectrum is shown in Table S11 of the Supporting Information, and the A-reduction spectroscopic constants obtained from 126 lines are displayed in Table 4. It is seen that the root-mean-square deviation of the fit is as large as 0.549 MHz. The perturbations from Watson’s Hamiltonian are of a different nature in AP than those encountered for SC. There are no closely spaced doublets, which was characteristic for the spectrum of SC. Instead, transitions are displaced up to several megahertz from the frequencies predicted from the Watson Hamiltonian. It is not obvious what causes the perturbations of the spectrum of AP. There are two low-vibrational frequencies, whose anharmonic frequencies are 150.6 and 170.6 cm−1 G
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predict that this form is 8.06 kJ/mol lower in energy than a rotamer with an anti-periplanar H−S−C−C link of atoms and no H bond.21 The H bond in HSCHCHCN is therefore much stronger than the H bonds in HSCH2CH2CN17 and in HSCH2CH2CCH.20
(Table S2). A coupling between these two states is a possibility. The barrier separating AP from SC is as low as 2.72 kJ/mol making interaction across this low barrier another possibility. It is interesting to compare the experimental and theoretical r0-rotational constants of Table 4. It is seen that the experimental A0 constant is smaller than the theoretical constant by 0.39%, whereas B0 and C0 are larger by 0.38 and 0.21%, respectively, which is satisfactory. The same cannot be said about all experimental quartic centrifugal distortion constants. ΔJ and δJ do not deviate very much from the theoretical values, but this is not the case for ΔJK, ΔK, and δK, where large differences are found. The two experimental sextic centrifugal distortion constants, ΦJ and ΦKJ, do not resemble the theoretical constants in Table 4 at all. The aforementioned perturbations in the spectrum of AP are presumed to be the cause of unrealistic effective values for ΔJK, ΔK, and δK. The different behaviors of the quartic centrifugal distortion constants in AP and SC presumably reflect different vibration−rotation interaction in the two conformers. It is seen from Table 4 that the value of twice the second moment, 2Pcc, is 3.0034(2) u Å2, nearly the same as 2.9769 and 2.9750 u Å2 found for the two isotopologues 35ClCH2CN and 37 ClCH2 CN, respectively, which are isoelectronic with HSCH2CN and known to have Cs symmetry.65 The value 3.13 u Å2 computed from the theoretical rotational constants of Table 4 is also in good agreement with the experimental value. A hump at the symmetry plane of AP would have led to a larger value than 3.0034(2) u Å2 for 2Pcc. The value of 2Pcc thus corroborates the CCSD and MP2 conclusions above that AP indeed has Cs symmetry. Energy Difference. The internal energy difference between the 0+ state of SC and the ground state of AP was determined in the manner described by Esbitt and Wilson66 by measuring relative intensities of selected bQ-transitions of each conformer. The energy difference was calculated as described by Townes and Schawlow.67 The CCSD dipole moment components (Table 1) were employed, since no experimental values are available. SC was found to be 3.8(3) kJ/mol lower in energy than AP. The uncertainty of ±0.3 kJ/mol represents one standard deviation. The experimental energy difference should be compared to the MP2 electronic energy difference of 4.22 kJ/mol, this energy difference corrected for zero-point vibrational energies (4.68 kJ/mol), and the CCSD electronic energy difference of 4.70 kJ/mol (see above). The experimental energy difference thus tends to be a little lower than the theoretical values. It would have been of interest to compare the present energy difference of SC and AP of HSCH2CN with its counterpart in the isoelectronic molecule HSCH2CCH to better understand the H-bond between a thiol group and π-electrons of a triple bond, but this information is not available because the microwave spectrum of the anti-periplanar form of HSCH2CCH has not been reported. However, the relative energies of the H-bond interaction of the thiol group with πelectrons of triple bonds have been determined experimentally in other cases. In HSCH2CH2CN,17 the H-bonded S−C− C−C synclinal conformer is 1.3(20) kJ/mol lower in energy than an anti-periplanar form without this interaction. In HSCH2CH2CCH,20 the energy difference between similar conformers is 1.7(4) kJ/mol with the H-bonded form as the energy minimum. Only one H-bonded conformer of Z-3mercapto-2-propenenitrile (HSCHCHCN) was found experimentally.21 However, MP2/aug-cc-pVTZ calculations
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DISCUSSION
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ASSOCIATED CONTENT
The following characteristics of the intramolecular H bond in SC were obtained from CCSD structure (Tables 1 and S1): The nonbonded H5···C2 distance is 287.7 pm, where the dots indicate a nonbonded interaction. This distance should be compared with 290 pm, which is the sum of the Pauling van der Waals radii of H (120 pm)68 and the half-thickness of an aromatic molecule (170 pm).68 The nonbonded H5···N3 distance is 365.1 pm in this conformer. The relatively longdistance form H5 to the π-electrons of the triple bond of nitrile group (C2N3) indicates that covalent forces must be comparatively weak. Electrostatic effects may also play a role for the H-bond strength. The bond moment of the thiol group (S−H) is 0.65 D with its negative end on the sulfur atom,69 whereas the nitrile group has a much larger bond moment of 3.6 D with its negative end on the nitrogen atom.69 The S4−H5 and C2N3 bonds are 60.0° from being parallel, as calculated from the CCSD structure (Table 1). The bond moments are consequently 60.0° from being antiparallel. Maximum stabilization would have occurred if the bond moments were exactly antiparallel. The fact that these bond moments are distorted by as much as 60.0° from this ideal geometry means that much, but not all, of the stabilizing electrostatic force has been lost. There is presumably a third force that stabilizes SC relative to AP. In ethanethiol (CH3CH2SH), the H−S−C−C synclinal conformer is lower in energy than the anti-periplanar form by 1.70(18) kJ/mol.70 This preference of the synclinal conformer in this compound has been attributed to an anomeric interaction.71 Weak covalent interaction between the thiol H atom and the π-electrons of the triple bond together with weak dipole− dipole stabilization of the thiol and cyano groups are assumed to be major contributors to the internal H bond in SC. If the anomeric interaction is responsible for ∼1.7 kJ/mol of the stabilization of SC relative to AP, the H bond should be responsible for another 2.1 kJ/mol making SC 3.8(3) kJ/mol lower in energy than AP.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b01600. Results of the theoretical calculations, including MP2/ccpVQZ harmonic and anharmonic vibrational frequencies, quartic and sextic centrifugal distortion constants, vibration rotation constants, re- and r0-rotational constants, and nuclear quadrupole coupling constants of the 14N nucleus. CCSD/cc-pV5Z electronic energies, structures, rotational constants, and dipole moments. UMP2/cc-pVTZ and MP2/aug-cc-pVTZ energies of several species. Microwave spectra of the parent and deuterated species of the SC and AP conformers. (PDF) H
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(14) Scappini, F.; Favero, P. G.; Cervellati, R. Microwave Double Resonance Assignment of the Roto-Torsional Spectrum of Propargyl Mercaptan. Chem. Phys. Lett. 1975, 33, 499−501. (15) Mirri, A. M.; Scappini, F.; Cervellati, R.; Favero, P. G. Microwave Spectrum of Deuteriopropargylmercaptan HCCCH2SD. Investigation of the Internal Rotation in the -SH and -SD Isotopic Species. J. Mol. Spectrosc. 1976, 63, 509−520. (16) Durig, J. R.; Klaassen, J. J.; Deodhar, B. S.; Gounev, T. K.; Conrad, A. R.; Tubergen, M. J. Microwave, Infrared, and Raman Spectra, r0 Structural Parameters, Conformational Stability, and Vibrational Assignment of Allylthiol. Spectrochim. Acta, Part A 2012, 87, 214−227. (17) Marstokk, K.-M.; Møllendal, H. Microwave Spectrum, Conformational Equilibrium, Intramolecular Hydrogen Bonding and Centrifugal Distortion of 3-Mercaptopropionitrile. Acta Chem. Scand. 1983, A37, 477−486. (18) Marstokk, K.-M.; Møllendal, H. Microwave Spectrum, Conformational Equilibria, Intramolecular Hydrogen Bonding and Centrifugal Distortion of 3-Butene-1-thiol. Acta Chem. Scand. 1986, A40, 402−411. (19) Marstokk, K.-M.; Møllendal, H. Microwave Spectra and Weak Intramolecular Hydrogen Bonding in 3-Butene-1-thiol and NMethylallylamine. NATO ASI Ser., Ser. C 1987, 212, 57−68. (20) Cole, G. C.; Møllendal, H.; Guillemin, J.-C. Microwave Spectrum of 3-Butyne-1-thiol: Evidence for Intramolecular S-H···π Hydrogen Bonding. J. Phys. Chem. A 2006, 110, 9370−9376. (21) Cole, G. C.; Møllendal, H.; Khater, B.; Guillemin, J.-C. Synthesis and Characterization of (E)- and (Z)-3-Mercapto-2-propenenitrile. Microwave Spectrum of the Z-Isomer. J. Phys. Chem. A 2007, 111, 1259−1264. (22) Møllendal, H.; Samdal, S.; Guillemin, J.-C. Microwave and Quantum Chemical Study of the Hydrazino Group as Proton Donor in Intramolecular Hydrogen Bonding of (2-Fluoroethyl)hydrazine (FCH2CH2NHNH2). J. Phys. Chem. A 2015, 119, 9252−9261. (23) Müller, H. S. P.; Schlöder, F.; Stutzki, J.; Winnewisser, G. The Cologne Database for Molecular Spectroscopy, CDMS: A Useful Tool for Astronomers and Spectroscopists. J. Mol. Struct. 2005, 742, 215− 227. (24) Brunken, S.; Gottlieb, C. A.; McCarthy, M. C.; Thaddeus, P. Laboratory Detection of HOCN and Tentative Identification in Sgr B2. Astrophys. J. 2009, 697, 880−885. (25) Frerking, M. A.; Linke, R. A.; Thaddeus, P. Interstellar Isothiocyanic Acid. Astrophys. J. 1979, 234, L143−L145. (26) Turner, B. E.; Kisliakov, A. G.; Liszt, H. S.; Kaifu, N. Microwave Detection of Interstellar Cyanamide. Astrophys. J. 1975, 201, L149− L152. (27) Remijan, A. J.; Hollis, J. M.; Lovas, F. J.; Stork, W. D.; Jewell, P. R.; Meier, D. S. Detection of Interstellar Cyanoformaldehyde (CNCHO). Astrophys. J. 2008, 675, L85−L88. (28) Zaleski, D. P.; Seifert, N. A.; Steber, A. L.; Muckle, M. T.; Loomis, R. A.; Corby, J. F.; Martinez, O., Jr.; Crabtree, K. N.; Jewell, P. R.; Hollis, J. M.; Lovas, F. J.; Vasquez, D.; Nyiramahirwe, J.; Sciortino, N.; Johnson, K.; McCarthy, M. C.; Remijan, A. J.; Pate, B. H. Detection of E-Cyanomethanimine Toward Sagittarius B2(N) in the Green Bank Telescope PRIMOS Survey. Astrophys. J., Lett. 2013, 765, L10 116 pp.. (29) Belloche, A.; Menten, K. M.; Comito, C.; Mueller, H. S. P.; Schilke, P.; Ott, J.; Thorwirth, S.; Hieret, C. Detection of Amino Acetonitrile in Sgr B2(N). Astron. Astrophys. 2008, 482, 179−196. (30) Turner, B. E.; Steimle, T. C.; Meerts, L. Detection of Sodium Cyanide (NaCN) in IRC 10216. Astrophys. J. 1994, 426, L97−L100. (31) Pulliam, R. L.; Savage, C.; Agundez, M.; Cernicharo, J.; Guelin, M.; Ziurys, L. M. Identification of KCN in IRC+10216: Evidence for Selective Cyanide Chemistry. Astrophys. J., Lett. 2010, 725, L181− L185. (32) Ziurys, L. M.; Apponi, A. J.; Guelin, M.; Cernicharo, J. Detection of MgCN in IRC + 10216: A New Metal-Bearing Free Radical. Astrophys. J. 1995, 445, L47−L50.
AUTHOR INFORMATION
Corresponding Author
*Phone: +47 2285 5674. Fax: +47 2285 5441. E-mail: harald.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been supported by the Research Council of Norway through a Centre of Excellence Grant (Grant No. 179568/V30). It has also received support from the Norwegian Supercomputing Program (NOTUR) through a grant of computer time (Grant No. NN4654K). J.-C.G. thanks the Centre National d’Etudes Spatiales and the Program Physique et Chimie du Milieu Interstellaire (INSU-CNRS) for grants.
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REFERENCES
(1) Møllendal, H. Microwave Spectrum, Conformation and Intramolecular Hydrogen Bonding of 2,2,2-Trifluoroethanethiol (CF3CH2SH). J. Phys. Chem. A 2008, 112, 7481−7487. (2) Møllendal, H. A Microwave and Quantum Chemical Study of Trifluorothioacetic Acid, CF3COSH, a Compound with an Unusual Double-Minimum Potential. J. Phys. Chem. A 2007, 111, 1891−1898. (3) Marstokk, K.-M.; Møllendal, H. Structural and Conformational Properties of 1,2-Ethanedithiol as Studied by Microwave Spectroscopy and Ab Initio Calculations. Acta Chem. Scand. 1997, 51, 653−663. (4) Marstokk, K.-M.; Møllendal, H.; Stenstrøm, Y. Conformational Composition in the Gas Phase and in Solution, Structure and Intramolecular Hydrogen Bonding of Thiiranemethanethiol as Studied by Microwave and NMR Spectroscopy and Ab Initio Calculations. Acta Chem. Scand. 1994, 48, 711−719. (5) Barkowski, S. L.; Hedberg, K. Conformational Analysis. 11. 2Aminoethanethiol. An Electron-Diffraction Investigation of the Molecular Structure, Conformational Composition, and Anti-Gauche Energy and Entropy Differences; Evidence for an Intramolecular Thiol-Nitrogen Hydrogen Bond. J. Am. Chem. Soc. 1987, 109, 6989− 6994. (6) Nandi, R. N.; Boland, M. F.; Harmony, M. D. Microwave Spectral Studies of Rotational Isomerism in Substituted Ethanethiols: 2Aminoethanethiol and 2-Chloroethanethiol. J. Mol. Spectrosc. 1982, 92, 419−430. (7) Caminati, W.; Velino, B.; Schäfer, L.; Ewbank, J. D.; Siam, K. Additional Evidence Concerning the Conformational Equilibrium of 2Aminoethanethiol from the Microwave Spectrum of the ND2-SD Trideuterated Species and Ab Initio Calculations. J. Mol. Struct. 1989, 197, 123−129. (8) Fantoni, A. C.; Caminati, W. A Double Minimum Motion and - S - H···OC Hydrogen Bond in Methylthioglycolate. J. Mol. Spectrosc. 1990, 143, 389−391. (9) Møllendal, H.; Khater, B.; Guillemin, J.-C. Synthesis, Microwave Spectrum, and Conformational Equilibrium of Propa-1,2-dienethiol (H2CCCHSH). J. Phys. Chem. A 2009, 113, 5906−5911. (10) Marstokk, K.-M.; Møllendal, H. Microwave Spectrum, Conformational Equilibrium, Intramolecular Hydrogen Bonding and Ab Initio Calculations for 2-Furanmethanethiol (Furfuryl Mercaptan). Acta Chem. Scand. 1994, 48, 298−305. (11) Marstokk, K.-M.; Møllendal, H. A Microwave and Ab Initio Study of the Conformational Properties and Intramolecular Hydrogen Bonding of Cyclopropanemethanethiol. Acta Chem. Scand. 1991, 45, 354−360. (12) Bolton, K.; Sheridan, J. Microwave Spectrum of Propargyl Mercaptan; Observation of Predominantly Vibrational Transitions of the Torsion Mode. Spectrochim. Acta, Part A 1970, 26, 1001−1006. (13) Scappini, F.; Mäder, H.; Sheridan, J. Study of the Internal Rotation in Propargyl Mercaptan by Microwave Spectrum Analysis. Z. Naturforsch., Teil A 1973, 28, 77−81. I
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The Journal of Physical Chemistry A
(53) Watson, J. K. G.: Vibrational Spectra and Structure; Elsevier: Amsterdam, 1977; Vol. 6. (54) Agúndez, M.; Fonfria, J. P.; Cernicharo, J.; Pardo, J. R.; Guélin, M. Detection of Circumstellar CH2CHCN, CH2CN, CH3CCH, H2CS. Astron. Astrophys. 2008, 479, 493−501. (55) Gardner, F. F.; Höglund, B.; Shukre, C.; Stark, A. A.; Wilson, T. L. Observations of Ortho- and Para-Thioformaldehyde. Astron. Astrophys. 1985, 146, 303−306. (56) Minowa, H.; Satake, M.; Hirota, T.; Yamamoto, S.; Ohishi, M.; Kaifu, N. Laboratory Microwave Spectroscopy of HDCS and its Astronomical Detection Toward TMC. Astrophys. J. 1997, 491, L63− L66. (57) Marcelino, N.; Cernicharo, J.; Roueff, E.; Gerin, M.; Mauersberger, R. Deuterated Thioformaldehyde in the Barnard 1 Cloud. Astrophys. J. 2005, 620, 308−320. (58) Geballe, T. R.; Oka, T. Detection of H3+ in Interstellar Space. Nature (London, U. K.) 1996, 384, 334−335. (59) Sørensen, G. O. Centrifugal Distortion Analysis of Microwave Spectra of Asymmetric Top Molecules. The Microwave Spectrum of Pyridine. J. Mol. Spectrosc. 1967, 22, 325−346. (60) Ray, B. S. The Characteristic Values of an Asymmetric Top. Eur. Phys. J. A 1932, 78, 74−91. (61) Wodarczyk, F. J.; Wilson, E. B., Jr. Radio Frequency-Microwave Double Resonance as a Tool in the Analysis of Microwave Spectra. J. Mol. Spectrosc. 1971, 37, 445−463. (62) Harris, D. O.; Harrington, H. W.; Luntz, A. C.; Gwinn, W. D. Microwave Spectrum, Vibration-Rotation Interaction, and Potential Function for the Ring-Puckering Vibration of Trimethylene Sulfide. J. Chem. Phys. 1966, 44, 3467−3480. (63) Nielsen, C. J. Centrifugal Distortion in Thietane. Acta Chem. Scand. 1977, A31, 791−792. (64) Kraitchman, J. Determination of Molecular Structure From Microwave Spectroscopic Data. Am. J. Phys. 1953, 21, 17−24. (65) Kisiel, Z.; Pszczolkowski, L. The Millimeter-Wave Rotational Spectrum of Chloroacetonitrile. J. Mol. Spectrosc. 1993, 158, 318−327. (66) Esbitt, A. S.; Wilson, E. B. Relative Intensity. Rev. Sci. Instrum. 1963, 34, 901−907. (67) Townes, C. H.; Schawlow, A. L. Microwave Spectroscopy; McGraw-Hill: New York, 1955. (68) Pauling, L. The Nature of the Chemical Bond; Cornell University Press: Ithaca, NY, 1960. (69) Exner, O. Dipole Moments in Organic Chemistry; Thieme, 1975. (70) Schmidt, R. E.; Quade, C. R. Microwave Spectrum of Ethyl Mercaptan. J. Chem. Phys. 1975, 62, 3864−3874. (71) Plant, C.; Spencer, K.; Macdonald, J. N. The Conformations of Some Simple Alcohols and Thiols. J. Chem. Soc., Faraday Trans. 2 1987, 83, 1411−1425.
(33) Zack, L. N.; Halfen, D. T.; Ziurys, L. M. Detection of FeCN (X4Δi) in IRC+10216: A New Interstellar Molecule. Astrophys. J., Lett. 2011, 733, L36. (34) Guelin, M.; Muller, S.; Cernicharo, J.; Apponi, A. J.; McCarthy, M. C.; Gottlieb, C. A.; Thaddeus, P. Astronomical Detection of the Free Radical SiCN. Astron. Astrophys. 2000, 363, L9−L12. (35) Anderson, J. K.; Ziurys, L. M. Detection of CCN (X2Πr) in IRC + 10216: Constraining Carbon-Chain Chemistry. Astrophys. J., Lett. 2014, 795, L1 6 pp.. (36) Sinclair, M. W.; Fourikis, N.; Ribes, J. C.; Robinson, B. J.; Brown, R. D.; Godfrey, P. D. Detection of Interstellar Thioformaldehyde. Aust. J. Phys. 1973, 26, 85−91. (37) Neufeld, D. A.; Falgarone, E.; Gerin, M.; Godard, B.; Herbst, E.; des Forets, G. P.; Vasyunin, A. I.; Guesten, R.; Wiesemeyer, H.; Ricken, O. Discovery of Interstellar Mercapto Radicals (SH) with the GREAT Instrument on SOFIA. Astron. Astrophys. 2012, 542, L6. (38) Agundez, M.; Cernicharo, J.; Guelin, M.; Kahane, C.; Roueff, E.; Klos, J.; Aoiz, F. J.; Lique, F.; Marcelino, N.; Goicoechea, J. R.; Gonzalez Garcia, M.; Gottlieb, C. A.; McCarthy, M. C.; Thaddeus, P. Astronomical Identification of CN−, the Smallest Observed Molecular Anion. Astron. Astrophys. 2010, 517, L2. (39) Irvine, W. M.; Friberg, P.; Hjalmarson, A.; Ishikawa, S.; Kaifu, N.; Kawaguchi, K.; Madden, S. C.; Matthews, H. E.; Ohishi, M.; et al. Identification of the Interstellar Cyanomethyl Radical (CH2CN) in the Molecular Clouds TMC-1 and Sagittarius B2. Astrophys. J. 1988, 334, L107−L111. (40) Thaddeus, P.; Kutner, M. L.; Penzias, A. A.; Wilson, R. W.; Jefferts, K. B. Interstellar Hydrogen Sulfide. Astrophys. J. 1972, 176, L73−L76. (41) Menten, K. M.; Wyrowski, F.; Belloche, A.; Guesten, R.; Dedes, L.; Mueller, H. S. Submillimeter Absorption from SH+, a New Widespread Interstellar Radical, 13CH+ and HCl. Astron. Astrophys. 2011, 525, A77. (42) Atacama Large Millimeter/submillimeter Array; http://www. almaobservatory.org/. (43) Gaumont, A. C.; Wazneh, L.; Denis, J. M. Thiocyanohydrins, a New Class of Compounds, Precursors of Unstabilized Thiocarbonyl Derivatives. Tetrahedron 1991, 47, 4927−4940. (44) Samdal, S.; Grønås, T.; Møllendal, H.; Guillemin, J.-C. Microwave Spectrum and Conformational Properties of 4-Isocyano1-butene (H2CCHCH2CH2NC). J. Phys. Chem. A 2014, 118, 1413−14099. (45) Møller, C.; Plesset, M. S. Note on the Approximation Treatment for Many-Electron Systems. Phys. Rev. 1934, 46, 618−622. (46) Cizek, J. Use of the Cluster Expansion and the Technique of Diagrams in Calculations of Correlation Effects in Atoms and Molecules. Adv. Chem. Phys. 1969, 14, 35−89. (47) Scuseria, G. E.; Janssen, C. L.; Schaefer, H. F., III. An Efficient Reformulation of the Closed-Shell Coupled Cluster Single and Double Excitation (CCSD) Equations. J. Chem. Phys. 1988, 89, 7382−7387. (48) Scuseria, G. E.; Schaefer, H. F., III. Is Coupled Cluster Singles and Doubles (CCSD) More Computationally Intensive Than Quadratic Configuration Interaction (QCISD)? J. Chem. Phys. 1989, 90, 3700−3703. (49) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision B.01; Gaussian, Inc: Wallingford, CT, 2010. (50) Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M.; et al. MOLPRO, a Package of Ab Initio Programs, version 2010.1; University College Cardiff Consultants Limited: Cardiff, Wales, U.K., 2010. http://www.molpro.net/. (51) Dunning, T. H., Jr. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron Through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (52) Peterson, K. A.; Dunning, T. H., Jr. Accurate Correlation Consistent Basis Sets for Molecular Core-Valence Correlation Effects: The Second Row Atoms Al-Ar, and the First Row Atoms B-Ne Revisited. J. Chem. Phys. 2002, 117, 10548−10560. J
DOI: 10.1021/acs.jpca.6b01600 J. Phys. Chem. A XXXX, XXX, XXX−XXX
