Asymmetric CH Stretch Research Articles

Infrared Activated Signatures and Jahn-Teller Dynamics of NO-CH4 Collision Complexes.

Bimolecular collision outcomes sensitively depend on the chemical functionality and relative orientations of the colliding partners that define the accessible reactive and nonreactive pathways. Accurate predictions from multidimensional potential energy surfaces demand a full characterization of the available mechanisms. Therefore, there is a need for experimental benchmarks to control and characterize the collision conditions with spectroscopic accuracy to accelerate the predictive modeling of chemical reactivity. To this end, the bimolecular collision outcomes can be investigated systematically by preparing reactants in the entrance channel prior to reaction. Herein, we investigate the vibrational spectroscopy and infrared-driven dynamics of the bimolecular collision complex between nitric oxide and methane (NO-CH4). We recorded the vibrational spectroscopy of NO-CH4 in the CH4 asymmetric stretching region using resonant ion-depletion infrared spectroscopy and infrared action spectroscopy, thus revealing a significantly broad spectrum centered at 3030 cm-1 that extends over 50 cm-1. The asymmetric CH stretch feature of NO-CH4 is explained by CH4 internal rotation and attributed to transitions involving three different nuclear spin isomers of CH4. The vibrational spectra also show extensive homogeneous broadening due to the ultrafast vibrational predissociation of NO-CH4. Additionally, we combine infrared activation of NO-CH4 with velocity map imaging of NO (X2Π, ν″ = 0, J″, Fn, Λ) products to develop a molecular-level understanding of the nonreactive collisions of NO with CH4. The anisotropy of the ion image features is largely determined by the probed rotational quantum number of NO (J″) products. For a subset of NO fragments, the ion images and total kinetic energy release (TKER) distributions show an anisotropic component at low relative translation (∼225 cm-1) indicating a prompt dissociation mechanism. However, for other detected NO products, the ion images and TKER distributions are bimodal, in which the anisotropic component is accompanied by an isotropic feature at high relative translation (∼1400 cm-1) signifying a slow dissociation pathway. In addition to the predissociation dynamics following vibrational excitation, the Jahn-Teller dynamics prior to infrared activation need to be considered to fully describe the product spin-orbit distributions. Therefore, we correlate the Jahn-Teller mechanisms of NO-CH4 to the symmetry-restricted NO (X2Π, ν″ = 0, J″, Fn, Λ) + CH4 (ν″) product outcomes.

Structural sensitivity of C[sbnd]H vibrational band in methyl benzoate

The CH vibrational bands of methyl benzoate are studied to understand its coupling pattern with other vibrational bands of the biological molecule. This will facilitate to understand the biological structure and dynamics in spectroscopic as well as in microscopic study. Due to the congested spectroscopic pattern, near degeneracy, and strong anharmonicity of the CH stretch vibrations, assignment of the CH vibrational frequencies are often misleading. Anharmonic vibrational frequency calculation with multidimensional potential energy surface interprets the CH vibrational spectra more accurately. In this article we have presented the importance of multidimensional potential energy surface in anharmonic vibrational frequency calculation and discuss the unexpected red shift of asymmetric CH stretch vibration of methyl group. The CD stretch vibrational band which is splitted to double peaks due to the Fermi resonance is also discussed here.

Doppler-limited high-resolution spectrum and VPT2 assisted assignment of the C-H stretch of CH2Br2

The Doppler limited non-saturated rotationally resolved infrared spectra of the symmetric and asymmetric CH-stretch bands of CH2Br2 have been measured. A continuous wave cavity ringdown setup with a widely tunable Mid-IR-OPO laser light source yielded a single-shot minimum absorption of 4.9×10−8cm−1. In contrast to the heavily congested ν1 band, the ν6 band showed partially resolved rotational features that may serve as suitable absorption targets in future environmental detection schemes for CH2Br2. A straightforward, VPT2 (second-order vibrational perturbation theory) assisted quantum-chemical approach for assigning the rotational structure has been tested using different model chemistries. The molecular structures, anharmonic frequencies and the structural changes upon vibrational excitation of CH2Br2 have been investigated. The predicted changes of the anharmonic rotational constants have been used together with available spectroscopic ground state constants to simulate the rovibrational structures of the ν1 and ν6 bands of CH2Br2. A refined analysis of the ν6 band is presented yielding accurate values for the band origin and the rotational constants. A fit of the line positions of 312 prominent transitions of the three isotopologues revealed a low standard error of 0.00056cm−1, hence within the absolute 0.0009cm−1 wavelength accuracy of the used spectrometer setup. A combined analysis of the predicted line strengths and positions of the strong Q sub-branches of the ν6 band has been performed to test the ability of the different density functionals for VPT2 prediction of anharmonic molecular constants. The M06/6-311++G(d,p) model chemistry turned out to yield reliable state-dependent rotational constants that are accurate enough to reproduce the overall rotational structure even without fitting.

Communication: conical intersections between vibrationally adiabatic surfaces in methanol.

A set of seven conical intersections (CI's) in methanol between vibrationally adiabatic surfaces is reported. The intersecting surfaces represent the energies of the two asymmetric CH stretch vibrations regarded as adiabatic functions of the torsion and COH bend angles. The ab initio data are well described by an extended Zwanziger and Grant (E ⊗ e) model [J. W. Zwanziger and E. R. Grant, J. Chem. Phys. 87, 2954 (1987)] that might also be regarded as an extension of the XHL model [L.-H. Xu, J. T. Hougen, and R. M. Lees, J. Mol. Spectrosc. 293-294, 38 (2013)]. The CI's illuminate the role of geometric phase in methanol. More generally, they suggest the importance of energy transfer processes localized near the CI's.

Torsion–Inversion Tunneling Patterns in the CH-Stretch Vibrationally Excited States of the G12 Family of Molecules Including Methylamine

Two torsion-inversion tunneling models (models I and II) are reported for the CH-stretch vibrationally excited states in the G12 family of molecules. The torsion and inversion tunneling parameters, h(2v) and h(3v), respectively, are combined with low-order coupling terms involving the CH-stretch vibrations. Model I is a group theoretical treatment starting from the symmetric rotor methyl CH-stretch vibrations; model II is an internal coordinate model including the local-local CH-stretch coupling. Each model yields predicted torsion-inversion tunneling patterns of the four symmetry species, A, B, E1, and E2, in the CH-stretch excited states. Although the predicted tunneling patterns for the symmetric CH-stretch excited state are the same as for the ground state, inverted tunneling patterns are predicted for the asymmetric CH-stretches. The qualitative tunneling patterns predicted are independent of the model type and of the particular coupling terms considered. In model I, the magnitudes of the tunneling splittings in the two asymmetric CH-stretch excited states are equal to half of that in the ground state, but in model II, they differ when the tunneling rate is fast. The model predictions are compared across the series of molecules methanol, methylamine, 2-methylmalonaldehyde, and 5-methyltropolone and to the available experimental data.

Relative Phase Change of Nearby Resonances in Temporally Delayed Sum Frequency Spectra.

Surface-selective sum frequency generation (SFG) spectroscopy has been previously shown to benefit from a finite time delay between two input laser pulses, which suppresses the nonresonant background and improves spectral resolution. Here we demonstrate another consequence of the time delay in SFG: depending on the magnitude of the delay, nearby resonances (e.g., vibrational modes) can "flip" their relative phase, i.e., appear either in-phase or out-of-phase with one another, resulting in either constructive or destructive interference in SFG spectra. This is significant for interpretation of the SFG spectra, in particular because the sign of the resonant amplitude provides the absolute molecular orientation (up vs down) of the vibrational chromophore. We present results and model calculations for symmetric and asymmetric CH-stretch modes of the methyl-terminated Si(111) surface, showing that the phase flip occurs when the delay matches half-cycle of the difference frequency between the two modes.

Vibrational Relaxation of Normal and Deuterated Liquid Nitromethane

Anti-Stokes Raman scattering is used to monitor vibrational energy redistribution in the ambient temperature liquids nitromethane (NM-h3) and perdeuterated nitromethane (NM-d3) after ultrafast IR excitation of either the symmetric or asymmetric CH- or CD-stretch transitions. The instantaneous populations of most of the fifteen NM vibrations are determined with good accuracy, and a global fitting procedure with a master equation is used to fit all the data. The pump pulses excite not only CH- or CD-stretches but also certain combinations of bending and nitro stretching fundamentals. The coupled vibrations that comprise the initial state are revealed via the instantaneous rise of the anti-Stokes transients associated with each vibrational fundamental. In contrast to many other polyatomic liquids studied previously, there is little energy exchange among the CH-stretch (or CD-stretch) excitations, which is attributed to the nearly free rotation of the methyl group in NM. The vibrational cooling process, which is the multistep return to a thermalized state, occurs in three stages in both NM-h3 and NM-d3. In the first stage, the parent CH- or CD-stretch decays in a few picoseconds, exciting all lower-energy vibrations. In the second stage, the midrange vibrations decay in 10-15 ps, exciting the lower-energy vibrations. In the third stage, these lower-energy vibrations decay into the bath in tens of picoseconds. The initial excitations are thermalized in approximately 150 ps in NM-h3 and there is little dependence on which CH-stretch is excited. VC is somewhat faster in NM-d3 with more dependence on the initial CD-stretch, taking approximately 100 ps with symmetric CD-stretch excitation and approximately 120 ps with asymmetric CD-stretch excitation. Comparison is made with earlier nonequilibrium molecular dynamics simulations of VC [Kabadi, V. N.; Rice, B. M. Molecular dynamics simulations of normal mode vibrational energy transfer in liquid nitromethane. J. Phys. Chem. A 2004, 108, 532-540]. The simulations do a good job of reproducing the observed VC process and in addition they predicted the slow interconversion among CH-stretch excitations and the slower relaxation of the asymmetric CH-stretch now observed here.

Imaging the State-Specific Vibrational Predissociation of the C2H2−NH3Hydrogen-Bonded Dimer

The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded ammonia-acetylene dimer were studied following excitation in the asymmetric CH stretch. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the asymmetric CH stretch fundamental, ammonia fragments were detected by 2 + 1 REMPI via the B1E'' <-- X1A1' and C'1A1' <-- X1A1' transitions. The fragments' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational levels of ammonia with one or two quanta in the symmetric bend (nu2 umbrella mode) and were converted to rotational-state distributions of the acetylene co-fragment. The latter is always generated with one or two quanta of bending excitation. All the distributions could be fit well when using a dimer dissociation energy of D0 = 900 +/- 10 cm(-1). Only channels with maximum translational energy <150 cm(-1) are observed. The rotational excitation in the ammonia fragments is modest and can be fit by temperatures of 150 +/- 50 and 50 +/- 20 K for 1nu2 and 2nu2, respectively. The rotational distributions in the acetylene co-fragment pair-correlated with specific rovibrational states of ammonia appear statistical as well. The vibrational-state distributions, however, show distinct state specificity among channels with low translational energy release. The predominant channel is NH3(1nu2) + C2H2(2nu4 or 1nu4 + 1nu5), where nu4 and nu5 are the trans- and cis-bend vibrations of acetylene, respectively. A second observed channel, with much lower population, is NH3(2nu2) + C2H2(1nu4). No products are generated in which the ammonia is in the vibrational ground state or the asymmetric bend (1nu4) state, nor is acetylene ever generated in the ground vibrational state or with CC stretch excitation. The angular momentum (AM) model of McCaffery and Marsh is used to estimate impact parameters in the internal collisions that give rise to the observed rotational distributions. These calculations show that dissociation takes place from bent geometries, which can also explain the propensity to excite fragment bending levels. The low recoil velocities associated with the observed channels facilitate energy exchange in the exit channel, which results in statistical-like fragment rotational distributions.

CH stretch/internal rotor dynamics in ethyl radical: High-resolution spectroscopy in the CH3-stretch manifold

High-resolution IR absorption spectra of supersonically cooled ethyl radicals (Trot approximately 20 K) have been obtained in a slit supersonic jet discharge expansion, revealing first rotationally resolved data for CH-stretch excitation of the methyl group. Three different vibrational bands are observed, one parallel (k=0<--0) and two perpendicular (/k/1<--0), which for a nearly decoupled methyl rotor framework would correspond to symmetric and (nearly degenerate) asymmetric CH-stretch excitations. However, the splitting between the two asymmetric CH-stretch excitations is anomalously large (approximately 125 cm(-1)), signaling the presence of interactions between the CH2 radical moiety and the opposing CH bond on the methyl group. This suggests an improved zeroth-order vibrational description as an isolated CH stretch, strongly redshifted by hyperconjugation, with localized vibrations in the remaining CH bonds split into symmetric and asymmetric stretches. Such a dynamical picture highlights a remarkably strong coupling between methyl CH-stretch vibrations and C-C torsional geometry and begins to elucidate discrepancies with previous matrix observations.

IR/UV double resonant spectroscopy of the methyl radical: Determination of ν3 in the 3pz Rydberg state

IR+UV double resonant ion-dip and ion-enhancement spectroscopies are employed to study the nu3 asymmetric CH stretch vibration fundamental of CH3 in the ground and 3p(z) Rydberg electronic states. CH3 radical is synthesized in the supersonic jet expansion by flash pyrolysis of azomethane (CH3NNCH3) prior to the expansion. The Q band of the 3(1) (1) 3p(z)<--X transition of CH3, not detected by conventional UV resonantly enhanced multiphoton ionization (REMPI) spectroscopy, is determined to lie at 59,898 cm(-1) using IR+UV REMPI spectroscopy. Energy of the asymmetric CH stretch of CH3 in the 3p(z) Rydberg state, nu3(3p(z)), is 3087 cm(-1), redshifted by approximately 74 cm(-1) with respect to ground state nu3(X).

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.

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 (&amp;gt;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.

Large amplitude skeletal isomerization as a promoter of intramolecular vibrational relaxation in CH stretch excited hydrocarbons

The high resolution, slit jet cooled infrared v=1←0 methyl asymmetric stretch spectra of trans-2-butene and 1-butene are reported. Both of these molecules are singly unsaturated butene chains, have 30 vibrational degrees of freedom, and yield nearly equivalent vibrational state densities (ρvib≊200 states/cm−1) at CH stretch levels of excitation. The key difference between these two molecules is the presence of a large amplitude C–C–C skeletal torsional coordinate in 1-butene corresponding to a low barrier, internal isomerization pathway which is completely absent in trans-2-butene. The trans-2-butene asymmetric CH stretch (ν16) spectrum is fully discrete at 0.002 cm−1 resolution, and the coarse structure readily assigned to zero order rovibrational transitions (J′K′aK′c ← J″K″aK″c) in an asymmetric top. Fragmentation of these zero order transitions into spectral ‘‘clumps’’ of fine structure provides direct evidence for coupling of the CH stretch to vibrational bath states, but no evidence for loss of Ka′ and Kc′ as good quantum labels in the spectrum. The average density of coupled states is found directly from the spectrum to be 114 states/cm−1, i.e., on the order of 0.5 ρvib. In contrast to the behavior in trans-2-butene, the 1-butene v=1←0 methyl asymmetric stretch spectrum exhibits an essentially continuous absorption contour even at Trot=6 K and 0.002 cm−1 resolution. On closer inspection, the 1-butene spectral envelope exhibits reproducible, intramolecular vibrational relaxation (IVR) induced fine structure limited by apparatus resolution and characteristic of highly congested IVR coupling. Analysis of this fine structure indicates a density of coupled states on the order of 1 000–10 000 states/cm−1, i.e., 20–30-fold in excess of ρvib, and 1–2 orders of magnitude larger than observed in trans-2-butene. In order to model the degree of fine structure observed in the spectrum, this level of spectral congestion essentially requires complete mixing of all ρvib⋅(2J′+1) rovibrational states consistent with conservation of total energy and angular momentum. The qualitatively dramatic differences between 1-butene and trans-2-butene behavior support a simple model for strong vibration-rotation (V-R) coupling in the bath states due to large amplitude skeletal motion in the C–C–C torsional mode which greatly enhances the available state density for IVR. Hence, the presence of a low barrier, skeletal isomerization coordinate may prove to be a general, moiety specific promoter for IVR processes in CH stretch excited hydrocarbons.

High-resolution infrared spectroscopy of 2-fluoroethanol in a molecular beam

The high-resolution infrared spectrum of 2-fluoroethanol (2FE) in a molecular beam was obtained in the region of 2990–2977 cm−1. This spectral region contains the asymmetric CH stretch of the fluorinated carbon. Excitation of the CH stretch has previously been observed to photochemically isomerize 2FE from the Gg′ to the Tt conformation. The high-resolution spectrum of this transition provides the information necessary to quantitatively evaluate the amount of vibrational mode-coupling between the asymmetric CH stretch and the torsional reactive coordinate. Minimal amounts of vibrational mode coupling were observed in the spectrum which is consistent with the slow photoisomerization rate. The correlation of the amount of mode coupling and the isomerization rate supports the conclusion that mode-selective vibrational coupling plays an important role in the photochemical dynamics. It is further suggested that the strong intramolecular attractive interactions limit the magnitude of the vibrational mode coupling.

Evidence of vibrational mode coupling in 2-fluoroethanol via high-resolution infrared spectroscopy in a molecular beam

Abstract : The rotationally resolved high resolution spectrum of 2-fluoroethanol in a molecular beam has been measured in an effort to determine the extent of mode-selective vibrational coupling. The high resolution spectrum of an asymmetric CH stretch clearly demonstrates that mode-coupling is present in 2- fluoroethanol. The extent of mode-coupling is significantly less than would be predicted based on previous work on molecules of a similar size and density of states. The small amount of mode-coupling in the CH stretch is consistent with the slow photoisomerization rate of 2-fluoroethanol upon excitation of the CH stretch, and is evidence that supports mode-selective vibrational coupling in the isomerization reaction of 2-fluoroethanol.