Infrared laser spectroscopy of the CH3-HCN radical complex stabilized in helium nanodroplets

The CH3-HCN and CD3-HCN radical complexes have been formed in helium nanodroplets by sequential pickup of a CH3 (CD3) radical and a HCN molecule and have been studied by high-resolution infrared laser spectroscopy. The complexes have a hydrogen-bonded structure with C3v symmetry, as inferred from the analysis of their rotationally resolved nu = 1 <-- 0 H-CN vibrational bands. The A rotational constants of the complexes are found to change significantly upon vibrational excitation of the C-H stretch of HCN within the complex, DeltaA = A'-A" = -0.04 cm(-1) (for CH3-HCN), whereas the B rotational constants are found to be 2.9 times smaller than that predicted by theory. The reduction in B can be attributed to the effects of helium solvation, whereas the large DeltaA is found to be a sensitive probe of the vibrational averaging dynamics of such weakly bound systems. The complex has a permanent electric dipole moment of 3.1 +/- 0.2 D, as measured by Stark spectroscopy. A vibration-vibration resonance is observed to couple the excited C-H stretching vibration of HCN within the complex to the lower-frequency C-H stretches of the methyl radical. Deuteration of the methyl radical was used to detune these levels from resonance, increasing the lifetime of the complex by a factor of 2. Ab initio calculations for the energies and molecular parameters of the stationary points on the CN+CH4 --> HCN+CH3 potential-energy surface are also presented.     

自由基-分子反应:F+Propene(CH2CHCH3)的从头算研究

The radical-molecule reaction F+propene (CH2CHCH3) was studied in detail by using the Becke's three parameter Lee-Yang-Parr-B3LYP/6-311G(d,p) and coupled cluster with single, double, and triple excitationsCCSD(T)/6-311+G(2d,2p). It is shown that F+propene reaction mainly occurs through complex-formation mechanism: F attacks the double bond of propene leading to the formation of complex 1 and complex 2. As the two radical complexes are metastable, they can quickly dissociate to H+C3H5F, CH3+C2H3F and HF+C3H5. Based on the ab initio calculations, the CH3+C2H3F is the main channel, and the H elimination and HF forming channels also provide some contribution to products. The calculated values are in good agreement with the recently reported experimental results.     

Theoretical study of the structures, stability and vibrational spectra of the nitrous acid complexes with CH4

The structures, stability and vibrational spectra of the binary complexes CH4...HONO-trans and CH4...HONO-cis have been investigated using ab initio calculations at the SCF and MP2 levels with 6-311++G(d,p) basis set and B3LYP calculations with 6-31G(d,p) and 6-31+G(d,p) basis sets. Full geometry optimization was made for the complexes studied. It was established that the complex CH4...HONO-trans is more stable by 0.41 kcal mol(-1) than the complex CH4...HONO-cis. The accuracy of the ab initio calculations have been estimated by comparison between the predicted values of the vibrational characteristics (vibrational frequencies and infrared intensities) and the available experimental data. It was established, that the methods, used in this study are well adapted to the problem under examination. The predicted values with the B3LYP calculations are very near to the results, obtained with 6-311++G(d,p)/MP2. The changes in the vibrational characteristics of methane and trans-, cis-nitrous acid upon formation of the hydrogen bond show that the complexes CH4...HONO-trans and CH4...HONO-cis have geometry in which the OH group interacts with a methane molecule forming a single hydrogen bond. This fact is confirmed by relatively strong perturbation of the OH stretching vibration to lower frequencies and an increase of the infrared intensity of this vibration up to three times upon hydrogen bonding.     

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). II. Velocity map imaging studies

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following excitation in the 4ν(1) region (OH stretch overtone, near 13,600 cm(-1)) was studied using sliced velocity map imaging. A new vibrational band near 13,660 cm(-1) arising from interaction with the antisymmetric CH stretch was discovered for CH(2)OH. In CD(2)OH dissociation, D atom products (correlated with CHDO) were detected, providing the first experimental evidence of isomerization in the CH(2)OH ? CH(3)O (CD(2)OH ? CHD(2)O) system. Analysis of the H (D) fragment kinetic energy distributions shows that the rovibrational state distributions in the formaldehyde cofragments are different for the OH bond fission and isomerization pathways. Isomerization is responsible for 10%-30% of dissociation events in all studied cases, and its contribution depends on the excited vibrational level of the radical. Accurate dissociation energies were determined: D(0)(CH(2)OH → CH(2)O + H) = 10,160 ± 70 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,135 ± 70 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,760 ± 60 cm(-1).     

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). I. A theoretical study

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following the excitation of high OH stretch overtones is studied by quasi-classical molecular dynamics calculations using a global potential energy surface (PES) fitted to ab initio calculations. The PES includes CH(2)OH and CH(3)O minima, dissociation products, and all relevant barriers. Its analysis shows that the transition states for OH bond fission and isomerization are both very close in energy to the excited vibrational levels reached in recent experiments and involve significant geometry changes relative to the CH(2)OH equilibrium structure. The energies of key stationary points are refined using high-level electronic structure calculations. Vibrational energies and wavefunctions are computed by coupled anharmonic vibrational calculations. They show that high OH-stretch overtones are mixed with other modes. Consequently, trajectory calculations carried out at energies about ~3000 cm(-1) above the barriers reveal that despite initial excitation of the OH stretch, the direct OH bond fission is relatively slow (10 ps) and a considerable fraction of the radicals undergoes isomerization to the methoxy radical. The computed dissociation energies are: D(0)(CH(2)OH → CH(2)O + H) = 10,188 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,167 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,787 cm(-1). All are in excellent agreement with the experimental results. For CH(2)OH, the barriers for the direct OH bond fission and isomerization are: 14,205 and 13,839 cm(-1), respectively.     

Reaction dynamics of Ca(4s3d 1D2)+CH4-->CaH(X 2Sigma+)+CH3: reaction pathway and energy disposal for the CaH product

The reaction pathway for Ca(4s3d 1D2)+CH4-->CaH(X 2Sigma+)+CH3 has been investigated by using a pump-probe technique in combination with potential-energy surface (PES) calculations. The nascent product distributions of CaH have been characterized with Boltzmann rotational temperatures of 1013+/-102 and 834+/-70 K for the v=0 and 1 levels, respectively, and a Boltzmann vibrational temperature of 1313+/-173 K. The rotational and vibrational energy partitions in CaH have been estimated to be 461+/-45 and 252+/-15 cm(-1), respectively. According to the PES calculations, the pathway favors an insertion mechanism. Ca(3 1D2) approaches CH4 along an attractive potential surface in a C2v (or Cs) symmetry and then the collision complex undergoes nonadiabatic transition to the reactive ground-state surface. An Arrhenius plot shows a potential-energy requirement of 2695+/-149 cm(-1), which accounts for the endothermicity of 2930 cm(-1) for the reaction scheme. The Ca-C bond distance in the transition state structure is short enough to allow for tight orbital overlap between CaH and CH3. The strong coupling between the moieties renders the energy transfer sufficient from CaH into the CH3 radical. As compared to the Ca(4 1P1) reaction, the dissociation lifetime of the intermediate complex with less excess energy is prolonged so as to cause much less vibrational energy disposal into CaH.     

Strongly blue-shifted C-H stretches: interaction of formaldehyde with hydrogen fluoride clusters

The equilibrium structures, binding energies, and vibrational spectra of the cyclic, hydrogen-bonded complexes formed between formaldehyde, H(2)CO, and hydrogen fluoride clusters, (HF)(1< or =n < or =4), are investigated by means of large-scale second-order M?ller-Plesset calculations with extended basis sets. All studied complexes exhibit marked blue shifts of the C-H stretching frequencies, exceeding 100 cm(-1) for n = 2-4. It is shown that these blue shifts are, however, only to a minor part caused by blue-shifting hydrogen bonding via C-H...F contacts. The major part arises due to the structural relaxation of the H(2)CO molecule under the formation of a strong C=O...H-F hydrogen bond which strengthens as n increases. The close correlation between the different structural parameters in the studied series of complexes is demonstrated, and the consequences for the frequency shifts in the complexes are pointed out, corroborating thus the suggestion of the primary role of the C=O...H-F hydrogen bonding for the C-H stretching frequency shifts. This particular behavior, that the appearance of an increasingly stronger blue shift of the C-H stretching frequencies is mainly induced by the formation of a progressively stronger C=O...H-F hydrogen bond in the series of H(2)CO...(HF)(1< or =n < or =4), complexes and only to a lesser degree by the formation of the so-called blue-shifting C-H...F hydrogen bond, is rationalized with the aid of selected sections of the intramolecular H(2)CO potential energy surface and by performing a variety of structural optimizations of the H(2)CO molecule embedded in external, differently oriented dipole electric fields, and also by invoking a simple analytical force-field model.     

Study of the H-F stretching band in the absorption spectrum of (CH3)2O...HF in the gas phase

The absorption spectra of the (CH3)2O...HF complex in the range of 4200-2800 cm(-1) were recorded in the gas phase at a resolutions of 0.1 cm(-1) at T = 190-340 K. The spectra obtained were used to analyze their structure and to determine the temperature dependencies of the first and second spectral moments. The band shape of the (CH3)2O...HF complex in the region of the nu1(HF) stretching mode was reconstructed nonempirically. The nu1 and nu3 stretching vibrations and four bending vibrations responsible for the formation of the band shape were considered. The equilibrium geometry and the 1D-4D potential energy surfaces were calculated at the MP2 6-311++G(2d,2p) level with the basis set superposition error taken into account. On the basis of these surfaces, a number of one- and multidimensional anharmonic vibrational problems were solved by the variational method. Solutions of auxiliary 1D and 2D vibrational problems showed the strong coupling between the modes. The energy levels, transition frequencies and intensities, and the rotational constants for the combining vibrational states necessary to reconstruct the spectrum were obtained from solutions of the 4D problem (nu1, nu3, nu5(B2), nu6(B2)) and the 2D problem (nu5(B1), nu6(B1)). The theoretical spectra reconstructed for different temperatures as a superposition of rovibrational bands associated with the fundamental, hot, sum, and difference transitions reproduce the shape and separate spectral features of the experimental spectra. The calculated value of the nu1 frequency is 3424 cm(-1). Along with the frequencies and absolute intensities, the calculation yields the vibrationally averaged values of the separation between the centers of mass of the monomers Rc.-of-m., R(O...F), and r(HF) for different states. In particular, upon excitation of the nu1 mode, Rc.-of-m. becomes shorter by 0.0861 A, and r(HF) becomes longer by 0.0474 A.     

CH2BrCl在265 nm光解产生的氯甲基自由基的振动内能

利用离子速度影像技术研究了CH2BrCl在265nm附近的激光光解.利用2+1共振增强多光子电离分别获得光解产物Br(2P1/2)和Br(2P3/2)的离子速度图像,从而得出Br(2P1/2)和Br(2P3/2)的速度分布,以及光解碎片的总平动能分布.据此,运用角动量守恒碰撞模型获得了解离氯甲基自由基(·CH2Cl)的振动内能分布.研究结果表明:CH2BrCl+hv→Br(2P1/2)+CH2Cl通道产生的氯甲基自由基中被激发的振动模主要是v4、v3+v4、v2+v4和v2+v6;CH2BrCl+hv→Br(2P3/2)+CH2Cl通道产生的氯甲基自由基中被激发的振动模主要是v2+v6、v1+v3、v2+v5、v2+v3+v5和v1+v5;母体分子CH2BrCl在吸收光解光子后除有v5(CBrstretch)振动模被激发外,还有v7(CH2a-stretch)等其它振动模也被激发.     

Jet-cooled infrared spectroscopy in slit supersonic discharges: symmetric and antisymmetric CH2 stretching modes of fluoromethyl (CH2F) radical

The combination of shot noise-limited direct absorption spectroscopy with long-path-length slit supersonic discharges has been used to obtain first high-resolution infrared spectra for jet-cooled CH2F radicals in the symmetric (nu1) and antisymmetric (nu5) CH2 stretching modes. Spectral assignment has yielded refined lower- and upper-state rotational constants and fine-structure parameters from least-squares fits to the sub-Doppler line shapes for individual transitions. The rotational constants provide indications of large amplitude vibrational averaging over a low-barrier double minimum inversion-bending potential. This behavior is confirmed by high-level coupled cluster singles/doubles/triples calculations extrapolated to the complete basis set limit and adiabatically corrected for zero point energy. The calculations predict a nonplanar equilibrium structure (theta approximately 29 degrees, where theta is defined to be 180 degrees minus the angle between the C-F bond and the CH2 plane) with a 132 cm(-1) barrier to planarity and a vibrational bend frequency (nu(bend) approximately 276 cm(-1)), in good agreement with previous microwave estimates (nu(bend) = 300 (30) cm(-1)) by Hirota and co-workers [Y. Endo et al., J. Chem. Phys. 79, 1605 (1983)]. The nearly 2:1 ratio of absorption intensities for the symmetric versus antisymmetric bands is in good agreement with density functional theory calculations, but in sixfold contrast with simple local mode CH2 bond dipole predictions of 1:3. This discrepancy arises from a surprisingly strong dependence of the symmetric stretch intensity on the inversion bend angle and provides further experimental support for a nonplanar equilibrium structure.     

Axial and equatorial hydrogen-bond conformers between (CH2)3S and H(D)F: Fourier transform infrared spectroscopy and ab initio calculations

The coexistence of axial and equatorial hydrogen-bonded conformers of 1?:?1 (CH(2))(3)S-HF (and -DF) has been observed in the same adiabatic expansion of a supersonic jet seeded with argon and in a static absorption cell at room temperature. High level calculations computed the axial conformer to be the most stable one with a small energy difference with respect to the equatorial one, in full agreement with previous microwave experiments. On the grounds of band contour simulations of FTIR spectra and ab initio energetic and anharmonic vibrational calculations, two pairs of ν(s) HF donor stretching bands, observed in a series of jet-FTIR spectra at 3457.9 and 3480.5 cm(-1) have been respectively assigned to the axial and equatorial forms of the 1?:?1 complex. In the jet-FTIR spectra series with HF, the assignment of an additional broad band (about 200 cm(-1) higher in frequency with respect to ν(s)) to a 1?:?2 complex has been supported by theoretical investigations. Experimental detection of both axial and equatorial forms of a cyclic trimer has been confirmed by calculated energetic and vibrational properties. The nature of hydrogen bonding has been examined within topological frameworks. The energetic partitioning within the 1?:?1 dimers has been elucidated with SAPT techniques. Interestingly, the interconversion pathway between two 1?:?1 structures has been explored and it was seen that the formation of the 1?:?1 complex affects the interconversion barrier on the ring puckering motion. The band contour analysis of gas phase FTIR experiments provided a consistent set of vibrational frequencies and anharmonic coupling constants, in good agreement with ab initio anharmonic vibrational calculations. Finally, from a series of cell-FTIR spectra recorded at different partial pressures of (CH(2))(3)S and HF monomers, the absorption signal of the 1?:?1 complex could be isolated which enabled to estimate the equilibrium constant K(p) = 0.023 at 298 K for the dimerization.     

High-resolution infrared spectroscopy of jet-cooled vinyl radical: symmetric CH2 stretch excitation and tunneling dynamics

First high-resolution IR spectra of jet-cooled vinyl radical in the C-H stretch region are reported. Detailed spectral assignments and least squares fits to an A-reduction Watson asymmetric top Hamiltonian yield rotational constants and vibrational origins for three A-type bands, assigned to single quantum excitation of the symmetric CH(2) stretch. Two of the observed bands arise definitively from ground state vinyl radical, as rigorously confirmed by combination differences predicted from previous midinfrared CH(2) wagging studies of Kanamori et al. [J. Chem. Phys. 92, 197 (1990)] as well as millimeter wave rotation-tunneling studies of Tanaka et al. [J. Chem. Phys. 120, 3604 (2004)]. The two bands reflect transitions out of symmetric (0(+)) and antisymmetric (0(-)) tunneling levels of vinyl radical populated at 14 K slit-jet expansion temperatures. The band origins for the lower-lower (0(+)<--0(+)) and upper-upper (0(-)<--0(-)) transitions occur at 2901.8603(7) and 2901.9319(4) cm(-1), respectively, which indicates an increase in the tunneling splitting and therefore a decrease in the effective tunneling barrier upon CH(2) symmetric stretch excitation. The third A-type band with origin at 2897.2264(3) cm(-1) exhibits rotational constants quite close to (but at high-resolution distinguishable from) the vinyl radical ground state, consistent with a CH(2) symmetric stretch hot band built on one or more quanta of excitation in a low frequency vibration. The observed CH(2) symmetric stretch bands are in excellent agreement with anharmonically scaled high level density functional theory (DFT) calculations and redshifted considerably from previous low resolution assignments. Of particular dynamical interest, Boltzmann analysis indicates that the pair of 0(+) and 0(-) tunneling bands exhibits 1:1 nuclear spin statistics for K(a)=even:odd states. This differs from the expected 3:1 ratio for feasible exchange of the two methylenic H atoms but is consistent with a 4:4 ratio predicted for interchange between all three H atoms. This suggests the novel dynamical possibility of large amplitude "roaming" of all three H atoms in vinyl radical, promoted by high internal vibrational excitation arising from dissociative electron attachment in the discharge.     

Rovibronic bands of the A<--X transition of CH3OO and CD3OO detected with cavity ringdown absorption near 1.2-1.4 microm

We have recorded several rovibronic bands of CH3OO and CD3OO in their A<--X transitions in the range of 1.18-1.40 microm with the cavity ringdown technique. While the electronic origins for these species have been reported previously, many newly observed rovibronic bands are described here. The experimental vibrational frequencies (given as nu in the unit cm(-1) in this paper) for the COO bending (nu8) and COO symmetric stretching (nu7) modes in the A state are 378 and 887 cm(-1) for CH3OO, and 348 and 824 cm(-1) for CD3OO, respectively. In addition, two other vibrational frequencies were observed for the A state of CD3OO, namely, nu5 (954 cm(-1)) and nu6 (971 cm(-1)). These experimental vibrational frequencies for the A state of both CH3OO and CD3OO are in good agreement with predictions from quantum-chemical calculations at the UB3LYP/aug-cc-pVTZ level. The enhanced activity of the nu5 vibrational mode in CD3OO is rationalized by mode mixing with the nu7 mode, as supported by calculations of multidimensional Franck-Condon factors. In addition, many hot bands involving the methyl torsional mode (nu12) are observed for both normal and deuterated methyl peroxy. These bands include the "typical" sequence transitions and some "atypical" ones due to the nature of the eigenvalues and eigenfunctions which are a consequence of the low, but very different, torsional barriers in the X and A states. In addition, the 12(2)2 band in CH3OO and the 12(3)3 band in CD3OO show quite different structures than the origin bands, an effect which results from tunneling splittings comparable to the rotational contour.     

Laser-induced fluorescence and pure rotational spectroscopy of the CH2CHS (vinylthio) radical

Laser-induced fluorescence (LIF) excitation spectra of the B-X (2)A(") electronic transition of the CH(2)CHS radical, which is the sulfur analog of the vinoxy (CH(2)CHO) radical, were observed under room temperature and jet-cooled conditions. The LIF excitation spectra show very poor vibronic structures, since the fluorescence quantum yields of the upper vibronic levels are too small to detect fluorescence, except for the vibrationless level in the B state. A dispersed fluorescence spectrum of jet-cooled CH(2)CHS from the vibrationless level of the B state was also observed, and vibrational frequencies in the X state were determined. Precise rotational and spin-rotation constants in the ground vibronic level of the radical were determined from pure rotational spectroscopy using a Fourier-transform microwave (FTMW) spectrometer and a FTMW-millimeter wave double-resonance technique [Y. Sumiyoshi et al., J. Chem. Phys. 123, 054324 (2005)]. The rotationally resolved LIF excitation spectrum for the vibronic origin band of the jet-cooled CH(2)CHS radical was analyzed using the ground state molecular constants determined from pure rotational spectroscopy. Determined molecular constants for the upper and lower electronic states agree well with results of ab initio calculations.     

A direct dynamics trajectory study of F- + CH(3)OOH reactive collisions reveals a major non-IRC reaction path

A direct dynamics simulation at the B3LYP/6-311+G(d,p) level of theory was used to study the F- + CH3OOH reaction dynamics. The simulations are in excellent agreement with a previous experimental study (J. Am. Chem. Soc. 2002, 124, 3196). Two product channels, HF + CH2O + OH- and HF + CH3OO-, are observed. The former dominates and occurs via an ECO2 mechanism in which F- attacks the CH3- group, abstracting a proton. Concertedly, a carbon-oxygen double bond is formed and OH- is eliminated. Somewhat surprisingly this is not the reaction path, predicted by the intrinsic reaction coordinate (IRC), which leads to a deep potential energy minimum for the CH2(OH)2...F- complex followed by dissociation to HF + CH2(OH)O-. None of the direct dynamics trajectories followed this path, which has an energy release of -63 kcal/mol and is considerably more exothermic than the ECO2 path whose energy release is -27 kcal/mol. Other product channels not observed, and which have a lower energy than that for the ECO2 path, are F- + CO + H2 + H2O (-43 kcal/mol), F- + CH2O + H2O (-51 kcal/mol), and F- + CH2(OH)2 (-60 kcal/mol). Formation of the CH3OOH...F- complex, with randomization of its internal energy, is important, and this complex dissociates via the ECO2 mechanism. Trajectories which form HF + CH3OO- are nonstatistical events and, for the 4 ps direct dynamics simulation, are not mediated by the CH3OOH...F- complex. Dissociation of this complex to form HF + CH3OO- may occur on longer time scales.     

Ab initio and direct quasiclassical-trajectory study of the F+CH4-->HF+CH3 reaction

We present an electronic structure and dynamics study of the F+CH4-->HF+CH3 reaction. CCSD(T)/aug-cc-pVDZ geometry optimizations, harmonic-frequency, and energy calculations indicate that the potential-energy surface is remarkably isotropic near the transition state. In addition, while the saddle-point F-H-C angle is 180 degrees using MP2 methods, CCSD(T) geometry optimizations predict a bent transition state, with a 153 degrees F-H-C angle. We use these high-quality ab initio data to reparametrize the parameter-model 3 (PM3) semiempirical Hamiltonian so that calculations with the improved Hamiltonian and employing restricted open-shell wave functions agree with the higher accuracy data. Using this specific-reaction-parameter PM3 semiempirical Hamiltonian (SRP-PM3), we investigate the reaction dynamics by propagating quasiclassical trajectories. The results of our calculations using the SRP-PM3 Hamiltonian are compared with experiments and with the estimates of two recently reported potential-energy surfaces. The trajectory calculations using the SRP-PM3 Hamiltonian reproduce quantitatively the measured HF vibrational distributions. The calculations also agree with the experimental HF rotational distributions and capture the essential features of the excitation function. The results of the SRP semiempirical Hamiltonian developed here clearly improve over those using the two prior potential-energy surfaces and suggest that reparametrization of semiempirical Hamiltonians is a promising strategy to develop accurate potential-energy surfaces for reaction dynamics studies of polyatomic systems.     

Product pair correlation in CH3OH photodissociation at 157 nm: the OH + CH3 channel

The OH + CH(3) product channel for the photodissociation of CH(3)OH at 157 nm was investigated using the velocity map imaging technique with the detection of CH(3) radical products via (2+1) resonance-enhanced multiphoton ionization (REMPI). Images were measured for the CH(3) formed in the ground and excited states (v(2) = 0, 1, 2, and 3) of the umbrella vibrational mode and correlated OH vibrational state distributions were also determined. We find that the vibrational distribution of the OH fragment in the OH + CH(3) channel is clearly inverted. Anisotropic distributions for the CH(3) (v(2) = 0, 1, 2, and 3) products were also determined, which is indicative of a fast dissociation process for the C-O bond cleavage. A slower CH(3) product channel was also observed, that is assigned to a two-step photodissociation process, in which the first step is the production of a CH(3)O(X (2)E) radical via the cleavage of the O-H bond in CH(3)OH, followed by probe laser photodissociation of the nascent CH(3)O radicals yielding CH(3)(X (2)A(1), v = 0) products.     

Vibration-rotation energy pattern in acetylene: 13CH12CH up to 6750 cm-1

All known vibration-rotation absorption lines of 13CH12CH accessing levels up to 6750 cm-1 were gathered from the literature. They were fitted simultaneously to J-dependent Hamiltonian matrices exploiting the well known vibrational polyad or cluster block diagonalization, in terms of the pseudo-quantum-numbers Ns=v1+v2+v3 and Nr=5v1+3v2+5v3+v4+v5, and accounting also for l parity and ef symmetry properties. The anharmonic interaction coupling terms known to occur from a pure vibrational fit in this acetylene isotopologue [Robert et al., J. Chem. Phys. 123, 174302 (2005)] were included in the model. A total of 12 703 transitions accessing 158 different (v1v2v3v4v5,l4l5) vibrational states was fitted with a dimensionless standard deviation of 0.99, leading to the determination of 216 vibration-rotation parameters. The experimental data included very weak vibration-rotation transitions accessing 18 previously unreported states, some of them forming Q branches with very irregular patterns.     

Infrared spectroscopy of Li(+)(CH4)1Ar(n), n = 1-6, clusters

Infrared predissociation (IRPD) spectra of Li(+)(CH(4))(1)Ar(n), n = 1-6, clusters are reported in the C-H stretching region from 2800 to 3100 cm(-1). The Li(+) electric field perturbs CH(4) lifting its tetrahedral symmetry and gives rise to multiple IR active modes. The observed bands arise from the totally symmetric vibrational mode, v(1), and the triple degenerate vibrational mode, v(3). Each band is shifted to lower frequency relative to the unperturbed CH(4) values. As the number of argon atoms is increased, the C-H red shift becomes less pronounced until the bands are essentially unchanged from n = 5 to n = 6. For n = 6, additional vibrational features were observed which suggested the presence of an additional conformer. By monitoring different photodissociation loss channels (loss of three Ar or loss of CH(4)), one conformer was uniquely associated with the CH(4) loss channel, with two bands at 2914 and 3017 cm(-1), values nearly identical to the neutral CH(4) gas-phase v(1) and v(3) frequencies. With supporting ab initio calculations, the two conformers were identified, both with a first solvent shell size of six. The major conformer had CH(4) in the first shell, while the conformer exclusively present in the CH(4) loss channel had six argons in the first shell and CH(4) in the second shell. This conformer is +11.89 kJ/mol higher in energy than the minimum energy conformer at the MP2/aug-cc-pVDZ level. B3LYP/6-31+G* level vibrational frequencies and MP2/aug-cc-pVDZ level single-point binding energies, D(e) (kJ/mol), are reported to support the interpretation of the experimental data.