CH Stretch Vibrations Research Articles

Cl − – C 6 H 6 , Br−–C6H6, and I−–C6H6 anion complexes: Infrared spectra and ab initio calculations

Vibrational predissociation spectroscopy is used to obtain infrared spectra of the Cl−–C6H6, Br−–C6H6, and I−–C6H6 complexes in the region of the benzene CH stretch vibrations (2800–3200 cm−1). The infrared spectra of the three dimers are similar, each exhibiting several narrow bands (full width at half maximum <10 cm−1) that are only slightly redshifted from the absorptions of the free benzene molecule. Ab initio calculations predict that the most stable form of the three complexes is a planar C2v structure in which the halide is hydrogen bonded to two adjacent CH groups. The planar C2v structure in which the halide is linearly H bonded to a single CH group is predicted to be slightly less stable than the bifurcated form. Comparisons between experimental and theoretically predicted infrared spectra confirm that the bifurcated structure is indeed the most stable conformer for all three complexes. Ab initio calculations show that the electron density transfer from the halide to the benzene is not limited to the σ*(CH) orbitals adjacent to the halide, but extends to the σ domain of the benzene ring, consistent with the moderate shift of the CH stretch frequencies. The presence of weak satellite bands is explained in terms of Fermi resonances reminiscent of the benzene Fermi tetrad or hot bands involving the in-plane intermolecular bend vibration.

Real-Time Observation of Intra- and Intermolecular Vibrational Energy Flow of Selectively Excited Alkyl Iodides in Solution: The Effect of Chemical Substitution

Intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of two alkyl iodides (CF3CH2I and CH3CH2I) selectively excited in the two quanta overtone region of the CH stretch vibration (υCH = 2) were measured in real time in solution. In this study, we have focused on the effect of chemical substitution on the mechanisms and time scales of IVR and VET of this family of molecules. With a simple model, we have obtained global IVR and VET rate coefficients for both molecules. The magnitude and the variation of the relaxation rates upon chemical substitution provide evidence for a survival of hierarchical IVR in these solvated molecules, which is governed by specific low-order intramolecular interactions and which can be rationalized with a simple low-order coupling model. At the same time, the general assumption that VET is simply dominated by the lowest-frequency modes in a molecule is not supported.

Overtone spectroscopy of some benzaldehyde derivatives

Overtone spectrum of o, m and p-nitrobenzaldehydes and p-chlorobenzaldehyde has been studied in 2000–12000 cm−1 region. Vibrational frequencies and anharmonicity constants for aryl as well as alkyl CH stretch vibrations have been determined. We have also determined the internuclear distances for the aryl CH bond in the different molecules. The small variation observed in these distances is an indication of the substitution effect. It is observed that in the case of p-disubstituted benzens, the shift in aryl CH bond is proportional to sum of the Hammet σ of the substituents. However in the case of o-disubstituted benzenes it is only 80% of the para-substituted shift.

Two-dimensional spectroscopy and harmonically coupled anharmonic oscillators

Experimentally it is established that the fourth-order anharmonicity plays significant roles in many molecules. In high resolution spectroscopy, it is known that a picture of local (anharmonic) modes with harmonic couplings between modes gives rise to a quartic Darling–Dennison coupling between normal modes. It has been shown that this order of anharmonicity can be selectively studied via seventh-order off-resonant spectroscopy, which probes the three-time response function (J. Chem. Phys. 106 (1997) 1687). In this paper, we further explore the effect of the fourth-order anharmonicity in the seventh-order signal, or in the three-time response function (which can be complementary investigated via third-order infrared spectroscopy), when the anharmonicity causes mode mixing; we obtain a fairly compact analytical result and numerically present the signal from CH stretch vibrations in methylene chloride as two-dimensional (2D) contour maps. It is found that purely kinetic coupling between anharmonic local oscillators can give rise to cross-peaks in the 2D spectra.

CH-stretching overtone spectra of a fast rotating methyl group. I. Toluene C6D5CH3

The CH-stretching overtone spectra of the methyl group in gaseous toluene C6D5CH3 are recorded with conventional Fourier transform near infrared spectroscopy in the ΔvCH=1–4 regions and by intracavity laser photoacoustic spectroscopy in the ΔvCH=5 and 6 regions. All spectra exhibit a complex structure. They are analyzed with a theoretical model which takes into account, within the adiabatic approximation, the coupling of the anharmonic CH stretch vibrations, described by Morse potentials, with the quasifree internal rotation of the methyl group and with isoenergetic combination states involving the six angle deformation modes of the methyl group. Using uniquely determined canonical potential-energy and kinetic-energy matrices allows us to solve the problem of indeterminacy caused by the description of molecular vibrations in such redundant coordinates. A set of Fermi resonance parameters and their variation with the internal rotation coordinate are determined from the fitting of the ΔvCH=1–6 overtone spectra. At Δv=1 and 2, the vibrational energy is expressed in a normal mode basis. Fermi resonance phenomena involving mainly the HCH bending modes lead to strong intramolecular vibrational energy redistribution. At this energy, a Coriolis coupling through internal rotation, which gives rise to a characteristic asymmetric top vibration-rotation profile, further perturbs the vibrational modes perpendicular to the rotation axis. From the second overtone (Δv=3), the vibrational energy is described in a local mode basis and perturbations due to Fermi resonance vanish progressively up to Δv=5. At Δv=6, a strong spectral perturbation is again observed. But, the efficient Fermi resonance phenomena are now essentially related to combination states involving HCH and HCC bending combination modes. This simple calculation successfully describes the relative intensity and frequency of each peak within a given overtone.

Measurement of the absolute intensity of the fifth CH stretching overtone of benzene using cavity ring-down spectroscopy

We present measurements of the absolute absorption intensity of the weak fifth overtone of the CH-stretch vibration of benzene at 16 550 cm −1. By using cavity ring-down spectroscopy, it is possible to obtain directly the absolute value of the oscillator strength of the detected transition ( f=(5.9±0.8)×10 −11). For this purpose, we had to develop a means of heating the cavity mirrors. The influence of the mirror heating on the measured spectra is discussed.

Intermolecular interaction in the CH3+–He ionic complex revealed by ab initio calculations and infrared photodissociation spectroscopy

The infrared photodissociation spectrum of the degenerate asymmetric CH stretch (ν3) vibration of the CH3+–He ionic complex has been recorded. The rotational structure and vibrational frequency of the observed transition are consistent with a π-bonded C3v cluster geometry where the He ligand is attached to the 2pz orbital of the central C atom of CH3+. The intermolecular bond in the ground vibrational state is characterized by an averaged intermolecular separation of Rcm=2.18 Å. The origin of the ν3 vibration of the complex is slightly blue shifted (7 cm−1) compared to the monomer frequency, indicating that vibrational excitation is accompanied by a slight destabilization of the intermolecular bond. Ab initio calculations at the MP2/aug-cc-pVTZ# level of theory confirm that the π-bonded configuration corresponds to the global minimum structure of the complex (De=707 cm−1, Re=1.834 Å). The calculated intermolecular potential energy surface of this “disk-and-ball” ionic complex reveals substantial angular-radial couplings in the region of the global minimum, which account for the large discrepancy between vibrationally averaged and calculated equilibrium intermolecular separations, Rcm−Re=0.35 Å.

Vibrational overtone spectroscopy and internal dynamics in gaseous nitromethane NO2CH2D

The CH-stretching overtone spectra of the methyl group in gaseous nitromethane NO2CH2D have been recorded with conventional Fourier transform near-infrared spectroscopy in the ΔvCH=1–4 regions and by intracavity laser photoacoustic spectroscopy in the ΔvCH=5 and 6 regions. All spectra exhibit a complex structure; they have been analyzed with a theoretical model which takes into account, within the adiabatic approximation, the coupling of the anharmonic CH stretch vibrations, described by Morse potentials, with the quasifree internal rotation of the methyl group and with isoenergetic combination states involving methyl bending modes. Most of the parameters of this model, and their variation with the internal rotation coordinate (θ), are identical to ones used to account for the overtone spectra of the monohydrogenated and perhydrogenated species. Fermi resonance phenomena, also modeled with θ dependent parameters, lead to only weak intramolecular vibrational energy redistribution. This simple calculation successfully describes the relative intensity and frequency of each peak within a given overtone. The vibrational energy is seen to be localized at lower energy in the dihydrogenated compound [at the second overtone (v=3)] than in the perhydrogenated one [at the third overtone (v=4)]. The overtone vibrations obtained from the calculations can be considered as normal modes up to Δv=2 and as local modes from Δv=3 to 6. However, at Δv=3, the existence of a transitional regime, where normal and local modes coexist, can be demonstrated. The CH/CD interbond coupling shifts the overtone spectra toward high for some additional weaker features in the high overtone spectra (Δv=5 and 6).

Vibrational state control of bimolecular reactions

The influence of rotation and vibration on the reactivity and the dynamics of the reaction X+HCN(ν1,ν2,ν3,J)→HX+CN(v,J) with X=H, Cl has been studied. The HCN molecule is prepared in a specific rovibrational level by IR/VIS overtone excitation in the wavelength region 6500–18 000 cm−1. The H atoms are generated by laser photolysis of CH3SH at 266 nm, the Cl atoms are formed in the photodissociation of Cl2 at 355 nm. The CN products are probed quantum state specifically by laser-induced fluorescence (LIF). For low rotational states of HCN, the reactivity of Cl and H is independent of the initial rotational state. However, an enhancement in reactivity of the Cl+HCN reaction is observed when the time of rotation becomes comparable to the passing time of the Cl atom. The reaction of Cl as well as of the H atom with HCN shows strong mode specific behavior, implying a simple direct reaction mechanism, which is also supported from Rice–Ramsperger–Kassel–Marcus (RRKM) calculations. An increase in CH stretch vibration increases both the reaction rate and the CN product vibration. Channeling energy in CN stretch vibration has only a minor effect on the reactivity and the CN product vibration even decreases. Trajectory calculations of the H+HCN system agree with the experimental results. The dependence of reaction rates on reactant approach geometry is investigated by preparing aligned reactants using linear polarized light. The CN signal is markedly influenced by the prepared alignments (steric effect). The experimental results suggest that the reaction of hydrogen and chlorine atoms with vibrationally excited HCN proceeds mainly via a collinear transition state, but the cone of acceptance is larger for chlorine atoms.

Jet-cooled molecular radicals in slit supersonic discharges: Sub-Doppler infrared studies of methyl radical

A novel high-intensity source of jet-cooled molecular radicals is described based on the combination of (i) slit supersonic expansions with (ii) pulsed electric discharges. The electrode bias configuration effectively confines the discharge to a region upstream of the supersonic expansion, which results both in efficient rotational cooling (Trot≈25 K) and high radical densities (>1014/cm3). In conjunction with direct absorption laser probe methods, this discharge source provides a general technique for high-resolution IR studies of jet-cooled radicals. Performance of the slit discharge system is demonstrated on v=1←0 rovibrational transitions in jet-cooled OH radicals, which indicate sub-Doppler linewidths (Δν≈100 MHz) when probed along the slit expansion axis. The enhanced spectral resolution of the slit discharge geometry is utilized to probe the v3=1←0 asymmetric CH stretch vibration–rotation spectra of CH3 radical. Under sub-Doppler conditions, spin–rotation splittings are fully resolved and nuclear hyperfine splittings partially resolved in all of the transitions, permitting the first measure of Fermi contact interactions [af″=−65.5(9) MHz, εbb″=−354(5) MHz, af′=−65(2) MHz, εbb′=−353(2) MHz] and therefore both the sign and magnitude of spin-polarization effects for CH3 under isolated gas-phase conditions. The results permit direct comparison with high level ab initio calculations, and highlight a clear trend in spin-polarization effects between condensed and gas-phase behavior.

Internal dynamics contributions to the CH stretching overtone spectra of gaseous nitromethane NO2CH3

The methyl CH stretching overtone spectra of gaseous nitromethane NO2CH3 have been recorded with Fourier transform infrared conventional near infrared spectroscopy in the ΔvCH=1–4 regions and by intracavity laser photoacoustic spectroscopy in the ΔvCH=5 and 6 regions. They all exhibit a complex structure with, at ΔvCH=1 and 2, a characteristic asymmetric top vibration-rotation profile which vanishes as vibrational energy increases. In addition, in these two lower energy spectra, the perpendicular stretching vibrations exhibit a widely spaced fine structure profile resulting from a Coriolis coupling induced by the methyl internal rotation. These excited spectra have been analyzed with a theoretical model which takes into account, in the adiabatic approximation, the coupling of the anharmonic CH stretch vibrations, described by a Morse potential, with the quasi-free internal rotation of the methyl group and with isoenergetic combination states involving methyl bending modes. Till Δv=3, the theoretical treatment yields normal modes. From the third overtone, the vibrational energy is seen to be localized and thus the calculations proceed in a local mode basis from Δv=4–6. Most of the parameters of this model and their variation with the internal rotation coordinate θ are the same as those used to account for the overtone spectra of the monohydrogenated species. Fermi resonance phenomena, also modeled with θ dependent parameters, lead to only weak IVR localized at the second overtone where only the two first tiers are effective and at the third overtone where three tiers must be considered to give a good reproduction of the experimental spectral features. This simple calculation successfully describes the relative intensity and frequency of each peak within a given overtone.

Hybrid and gradient-corrected density functional theory computations of the cubane infrared and raman spectra

The IR and Raman spectra of cubane were computed with three hybrid (B3LYP, B3P86, and B3PW91) and three gradient-corrected (BLYP, BP86, and BPW91) Density Functional Theory (DFT) methods using a 6-31G(d) basis set. The computed CH and CC bond distances were in good agreement with experimental data. The closest agreement with the experimental IR and Raman spectra was obtained with the hybrid B3LYP method. The deviation from the experimental frequencies depended on the nature of the vibration. Thus, CCC-bend and CC-stretch vibrations were higher by 10–20 cm −1, CH-wagging vibrations were higher by 50–60 cm −1, and CH-stretch vibrations were higher by 160–170 cm −1 than the experimental values. By including correlation factors, excellent agreement between the B3LYP computed IR and Raman spectra with the experimental data was obtained.

Reaction dynamics calculations for the CN+H2→HCN+H reaction: Applications of the rotating-bond approximation

We present quantum dynamics studies of the CN+H2→HCN+H reaction. An extended version of the rotating bond approximation (RBA), in which nonreactive CN stretch motion is taken into account in the dynamics explicitly, has been employed, and we have used a potential energy surface that was recently developed by ter Horst, Schatz, and Harding [J. Chem. Phys. 105, 558 (1996)]. This surface describes the HCN force field quite accurately and has significant coupling between CH and CN stretch vibrations in HCN. We find that neither CN vibration nor rotation affect the reaction cross section for the forward reaction significantly. This result is consistent with the fact that the forward reaction has an early barrier. On the other hand, for the reverse reaction HCN+H, HCN vibrational excitation significantly lowers the reactive threshold and enhances the cross section at energies above threshold. We find for the reverse reaction that all three modes of HCN (CH stretch, CN stretch and bend) reduce the threshold by an amount which equals the energy put into reagent vibration, but the enhancement in the cross section at a fixed energy above threshold is largest for energy put into the CH stretch. We also find that the HCN vibrational state distributions for the CN+H2 reaction show significant population in both CH and CN stretching normal modes. These results indicate that the nonreactive CN bond in the CN+H2 reaction, as well as its reverse, cannot be described totally by a simple spectator model, and that coupling between CH and CN stretch vibrations plays an important role in the dynamics.

Improvements in a Curvilinear Internal Coordinate Model for the CH Stretching and Bending Vibrations in Trihalomethanes

The interactions between the CH stretching and bending vibrations in CH X 3-type symmetric-top molecules have been studied. The previous curvilinear internal coordinate vibrational Hamiltonian (E. Kauppi and L. Halonen, J. Chem. Phys. 90, 6980-6992, 1989) has been improved by including contributions arising from the nonlinearity of the redundancy relation between the curvilinear internal coordinates. This affects the anharmonic kinetic energy terms of the pure CH bending Hamiltonian. Vibrational energy levels have been calculated variationally. The model has been applied to data for CHF 3, CHCl 3, CHBr 3, and (CF 3) 3CH. The potential energy parameters have been optimized by a nonlinear least-squares method using vibrational term values as data. In the cases of CHF 3, CHCl 3, and CHBr 3 data for bath 12C and 13C isotopomers have been treated with one set of isotope invariant potential energy parameters. Good fits have been obtained by using five or six adjustable parameters. The values for the quartic bending force constants change considerably from the values obtained by ignoring the nonlinear redundancy contributions.

Configuration interaction studies on the S2surface of H2CO: 21A′(σ, π*/π, π*) as perturber of 11B2(n, 3s)

MRD-Cl calculations reveal that the third singlet surface S2 of H2CO exhibits two energetically close-lying minima. The more stable state at 7.09 eV is the experimentally known 1 1B2(n, 3s). The second minimum at 7.53 eV corresponds to the pyramidal state 2 1A′, of mixed σ, π* and π, π* character near equilibrium. This S2 valence minimum, which apparently has not been experimentally observed, is suggested to cause the blue shift of the S2â†� S0 system of H2CO in condensed media (as well as in related aldehydes and ketones), ruling out n →σ*co(1B2) as responsible for this blue shift. The 21A′, state is non-adiabatically coupled with the ground state, explaining the observed predissociation of 11B2 into the CH2+ O continuum. The planar (C2v) and pyramidal (Cs) minima can be connected only through a C1 intermediary, requiring participation of the antisymmetric CH-stretch and CH2 bend vibrations, besides the CO-stretch and out-of-plane modes. A dipole moment µ=+3.45 D (1D ≈ 3.335 64 × 10–30 C m) with polarity (H2C)–O+ is predicted for 11B2(n, 3s) at equilibrium, at variance with an experimental µ of +0.33 D. An experimental route for studying the valence region of S2 is proposed.

The effect of internal rotation on the methyl CH-stretching overtone spectra of toluene and the xylenes

The structure of methyl CH-stretching overtone bands in the vibrational spectra of methylbenzenes was investigated theoretically. The anharmonic CH-stretching vibration, described by a Morse potential, was represented in terms of a harmonic basis while hindered internal rotation of the methyl group was represented by a rigid rotor attached to an infinitely massive frame. Relatively weak coupling between the anharmonic CH vibration and the hindered internal rotation is sufficient to shift the positions of rovibrational lines from a PQR-like rotational contour to patterns similar to those observed experimentally. For high rotational barriers, as in o-xylene, the rovibrational transitions form two bands associated with conformationally nonequivalent CH-bonds, consistent with the conformational preference established by microwave spectroscopy and molecular orbital calculations. For nearly free internal rotation, as in toluene, m-xylene and p-xylene, a prominent middle band is also present. This ‘‘free rotor’’ band corresponds to rotational transitions between states high above the barrier and disappears as the barrier height increases. The outer bands correspond to transitions for which either the initial or the final state is below or near the barrier height in energy. Contrary to earlier suggestions, the band structure is not indicative of the conformational preference of the methyl group in toluene. In fact, the calculated spectra of nearly free internal rotors are insensitive to this preference.

Intramolecular vibrational redistribution of energy in the stimulated emission pumping spectrum of acetylene

Using a combination of low resolution dispersed Ã→X̃ fluorescence spectra and high resolution stimulated emission pumping, we have spectroscopically identified the first stages of vibrational energy flow in the highly vibrationally excited acetylene prepared by Ã→X̃ emission over the energy range 5 000–18 000 cm−1. A detailed study of the stimulated emission pumping (SEP) spectrum of acetylene in the EVIB=7000 cm−1 region, in which we report spectroscopic constants and rovibrational term values for 12 vibrational levels, has conclusively shown that Darling–Dennison resonance between the cis and trans degenerate bending vibrations is the first step in the redistribution of vibrational energy from the initially excited Franck–Condon bright CC stretch and trans-bend vibrational combination levels. This allows an extension of our prior dispersed fluorescence (DF) assignments which suggested the crucial role of Darling–Dennison coupling between the cis and trans bends in IVR [J. Chem. Phys. 95, 6336 (1991)]. We prove that the symmetric CH stretch vibration, previously thought to play a crucial role in the redistribution of vibrational energy, is Franck–Condon inactive. We have also shown that vibrational-l-resonance among the states with excitation of both degenerate bending modes, when combined with a Fermi resonance which couples CC stretch/trans/cis-bend excited states to the antisymmetric CH stretch, determines the subsequent flow of vibrational energy after the Darling–Dennison bending resonance. These resonances all scale with vibrational excitation in nearly the simple manner expected for the lowest order anharmonic terms in the Hamiltonian, which allows the prediction of the fastest processes at high energy from a detailed study of the high resolution spectrum at lower energy. We find some interesting rules for vibrational energy flow in the short time dynamics: (i) CC stretch excitation is necessary for stretch–bend coupling; (ii) if V2″ and V4″ are the quantum numbers of the initially excited bright state, and vb″ = v4″ + v5″ is the total bending quantum number of a state coupled to that bright state, then V4″ ≥ vb″ ≥ (V4″–2V2″); (iii) the total stretch quantum number ns″ = (v1″ + v2″ + v3″) is also conserved by the short time dynamics. These are severe and well characterized restrictions on the range of quantum numbers accessible to the initial bright state during the first stages of intramolecular vibrational redistribution of energy.

Theoretical study of intramolecular vibrational relaxation of acetylenic CH vibration for v=1 and 2 in large polyatomic molecules (CX3)3YCCH, where X=H or D and Y=C or Si

Quantum calculations are reported for the intramolecular vibrational energy redistribution and absorption spectra of the first two excited states of the acetylenic CH stretch vibration in the polyatomic molecules (CX3)3YCCH, where X=H or D and Y=C or Si. Using approximate potential energy surfaces, comparison is made with the corresponding recent experimental spectra. It is found that a model of intramolecular vibrational relaxation based on the assumption of sequential off-resonance transitions via third and fourth order vibrational couplings (as opposed to direct high order couplings) is in agreement with experimental results on spectral linewidths. In a semiclassical limit this type of relaxation corresponds to a dynamic tunneling in phase space. It is shown that the local density of resonances of third and fourth order, rather than the total density of states, plays a central role for the relaxation. It is found that in the Si molecule an accidental absence of appropriate resonances results in a bottleneck in the initial stages of relaxation. As a result, an almost complete localization of the initially prepared excitation occurs. It is shown that an increase of the mass alone of the central atom from C to Si cannot explain the observed difference in the C and Si molecules. The spectral linewidths were calculated with the Golden Rule formula after prediagonalization of the relevant vibrational states which are coupled in the molecule to the CH vibration, directly or indirectly. For the spectral calculations, in addition to the direct diagonalization, a modified recursive residue generation method was used, allowing one to avoid diagonalization of the transformed Lanczos Hamiltonian. With this method up to 30 000 coupled states could be analyzed on a computer with relatively small memory. The efficiency of C programming language for the problem is discussed.

IR study on double hydrogen bonding in dichloromethane

Evidence of double hydrogen-bond formation between dichloromethane and electron-donor molecules is given using IR spectroscopy. The analysis of IR spectra is based mainly on the intensity changes of the ν 1 and ν 6 CH-stretching vibration bands of dichloromethane and deuterated dichloromethane dissolved in several electron-donor solvents. Dimerisation of dichloromethane diluted in inert solvents was also observed.

The influence of ?communicating states? on the dissociation yield in theS 1 of benzene

Non-radiative communicating states rate calculations forS 1 ⇝S 0 transitions in benzene are presented. The contribution ofC 6 H 5+H predissociation to the non-radiative rates with varying excess energy is investigated. Local modes with Morse oscillators for CH-stretch vibrations are introduced. In the channel-three region one finds that much less than 1% of the initial excited energy is found to be of discrete continuum character.C 6 H 5+H vibrational predissociation does not conform to the strong increase of “pure internal conversion” for high excess energies. The steep decrease of the dissociation yield at still higher excess energies is found to be closely related to the energy acceptor behaviour of the (harmonic) out-of-plane modes.