Methyl Torsional Modes Research Articles

Large Amplitude Motions of Pyruvic Acid (CH3-CO-COOH).

Torsional and rotational spectroscopic properties of pyruvic acid are determined using highly correlated ab initio methods and combining two different theoretical approaches: Second order perturbation theory and a variational procedure in three-dimensions. Four equilibrium geometries of pyruvic acid, Tc, Tt, Ct, and CC, outcome from a search with CCSD(T)-F12. All of them can be classified in the Cs point group. The variational calculations are performed considering the three internal rotation modes responsible for the non-rigidity as independent coordinates. More than 50 torsional energy levels (including torsional subcomponents) are localized in the 406–986 cm−1 region and represent excitations of the ν24 (skeletal torsion) and the ν23 (methyl torsion) modes. The third independent variable, the OH torsion, interacts strongly with ν23. The A1/E splitting of the ground vibrational state has been evaluated to be 0.024 cm−1 as it was expected given the high of the methyl torsional barrier (338 cm−1). A very good agreement with respect to previous experimental data concerning fundamental frequencies (νCAL − νEXP ~ 1 cm−1), and rotational parameters (B0CAL − B0EXP < 5 MHz), is obtained.

Effects of Confinement and Pressure on the Vibrational Behavior of Nano-Confined Propane

Fluids confined in nanopores exhibit significant deviations in their structure and dynamics from the bulk behavior. Although phase, structural, and diffusive behaviors of confined fluids have been investigated and reported extensively, confinement effects on the vibrational properties are less understood. We study the vibrational behavior of propane confined in 1.5 nm nanopores of MCM-41-S using inelastic neutron scattering (INS) and molecular dynamics (MD) simulations. Vibrational spectra have been obtained from INS data as functions of temperature and pressure. At ambient pressure, a strong quasielastic signal observed in the INS spectrum at 80 K suggests that confined propane remains liquid below the bulk phase melting point of 85 K. The quasielastic signal is heavily suppressed when either the pressure is increased to 1 kbar or the temperature is lowered to 30 K, indicating solidification of pore-confined propane. Confinement in MCM-41-S pores results in a glass-like state of propane that exhibits a relatively featureless low-energy vibrational spectrum compared to that of the bulk crystalline propane. Increasing the pressure to 3 kbar results in hardening of the intermolecular and methyl torsional modes. The INS data are used for estimating the isochoric specific heat of confined propane, which is compared with the specific heat of bulk propane reported in literature. Data from MD simulations are used to calculate the vibrational power spectra that agree qualitatively with the experimental data. Simulation data also suggest a reduction of the structural ordering (positional, orientational, and intramolecular) of propane under confinement.

Vibrational spectroscopy and DFT calculations of 1,3-dibromo-2,4,6-trimethylbenzene: Anharmonicity, coupling and methyl group tunneling

The Raman, IR and INS spectra of 1,3-dibromo-2,4,6-trimethylbenzene (DBMH) were recorded in the 80–3200cm−1 range. The molecular conformation and vibrational spectra of DBMH were computed at the MPW1PW91/LANL2DZ level. Except for the methyl 2 environment, the agreement between the DFT calculations and the neutron diffraction structure is almost perfect (deviations<0.01Å for bond lengths, <0.2° for angles). The frequencies of the internal modes of vibration were calculated with the harmonic and anharmonic approximations; the later method yields results that are in remarkable agreement with the spectroscopic data, resulting in a confident assignment of the vibrational bands. Thus, no scaling is necessary. The coupling, in phase or anti-phase, of the motions of symmetrical CBr and CMe bonds is highlighted. Our DFT calculations suggest that the torsion of methyl groups 4 and 6 is hindered in deep wells, whereas methyl group 2 is a quasi-free rotor. The failure of the calculations to determine the frequencies of the methyl torsional modes is explained as follows: DFT does not consider the methyl spins and assumes localization of the protons, whereas the methyl groups must be treated as quantum rotors.

CCSD(T) Study of CD3–O–CD3 and CH3–O–CD3 Far-Infrared Spectra

From a vibrationally corrected 3D potential energy surface determined with highly correlated ab initio calculations (CCSD(T)), the lowest vibrational energies of two dimethyl-ether isotopologues, (12)CH(3)-(16)O-(12)CD(3) (DME-d(3)) and (12)CD(3)-(16)O-(12)CD(3) (DME-d(6)), are computed variationally. The levels that can be populated at very low temperatures correspond to the COC-bending and the two methyl torsional modes. Molecular symmetry groups are used for the classification of levels and torsional splittings. DME-d(6) belongs to the G(36) group, as the most abundant isotopologue (12)CH(3)-(16)O-(12)CH(3) (DME-h(6)), while DME-d(3) is a G(18) species. Previous assignments of experimental Raman and far-infrared spectra are discussed from an effective Hamiltonian obtained after refining the ab initio parameters. Because a good agreement between calculated and experimental transition frequencies is reached, new assignments are proposed for various combination bands corresponding to the two deuterated isotopologues and for the 020 → 030 transition of DME-d(6). Vibrationally corrected potential energy barriers, structural parameters, and anharmonic spectroscopic parameters are provided. For the 3N - 9 neglected vibrational modes, harmonic and anharmonic fundamental frequencies are obtained using second-order perturbation theory by means of CCSD and MP2 force fields. Fermi resonances between the COC-bending and the torsional modes modify DME-d(3) intensities and the band positions of the torsional overtones.

Torsional vibrational structure of the propene radical cation studied by high-resolution photoelectron spectroscopy

The pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the X̃(+2)A(")←X̃(1)A' transition of CH(3)CHCH(2) (propene), CD(3)CDCD(2), and several partially deuterated isotopomers have been recorded in the region of their adiabatic ionization thresholds and up to 2000 cm(-1) of internal energy of the cations. The vibrational structure has been assigned on the basis of the frequency shifts resulting from deuteration of selected sites of the propene molecule. Two highly anharmonic progressions have been identified and assigned to the two torsional modes of the propene cation, the methyl and methylene torsions. The positions of the torsional levels could be approximately reproduced using one-dimensional models, allowing a semi-quantitative description of the potential energy surface along each torsional coordinate. The observation of forbidden vibrational bands and the analysis of their partially resolved rotational contours reveal the importance of the vibronic coupling between the X̃(+2)A(") and the Ã(+2)A(') states mediated by the methylene (ν(20)) and methyl (ν(21)) torsional modes.

Intramolecular vibrational dynamics in S1 p-fluorotoluene. I. Direct observation of doorway states

Picosecond time-resolved photoelectron spectroscopy is used to investigate intramolecular vibrational redistribution (IVR) following excitation of S(1) 18a(1) in p-fluorotoluene (pFT) at an internal energy of 845 cm(-1), where ν(18a) is a ring bending vibrational mode. Characteristic oscillations with periods of 8 ps and 5 ps are observed in the photoelectron signal and attributed to coupling between the initially excited zero-order bright state and two doorway states. Values for the coupling coefficients connecting these three vibrational states have been determined. In addition, an exponential change in photoelectron signal with a lifetime of 17 ps is attributed to weaker couplings with a bath of dark states that play a more significant role during the latter stages of IVR. A tier model has been used to assign the most strongly coupled doorway state to S(1) 17a(1) 6a(2)('), where ν(17a) is a CH out-of-plane vibrational mode and 6a(2)(') is a methyl torsional level. This assignment signifies that a torsion-vibration coupling mechanism mediates the observed dynamics, thus demonstrating the important role played by the methyl torsional mode in accelerating IVR.

Analysis of the nitrile and methyl torsional vibrations of n‐propyl cyanide in its S0 electronic state

AbstractThis work presents a structural and vibrational theoretical study of n‐propyl cyanide as a function of the nitrile and methyl torsional modes. A potential energy hypersurface is built at the MP4(SDQ)/aug‐cc‐pVTZ//MP2/aug‐cc‐pVTZ theory level. The equilibrium structure is found in a gauche conformation. Another minimum is found for the trans form. The maximum appears in a cis conformation. For the first time, the interconversion barriers between the different forms are calculated. A two‐dimensional anharmonic vibrational Hamiltonian is built for the nitrile and methyl torsional modes. We find the vibrational energy levels to organize in two stacks associated to the gauche and trans forms. Fundamental frequencies of 113.12 and 220.54 cm−1 are predicted for the nitrile and methyl torsions in the equilibrium, gauche, conformer. In addition, we find symmetry allowed transitions between the gauche and trans energy levels stacks. The lowest transition is predicted to appear at 24.49 cm−1. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Analysis of the rotational spectrum of pyruvonitrile up to 324 GHz

The room-temperature rotational spectrum of pyruvonitrile (acetyl cyanide, CH 3COCN) was measured up to 324 GHz, and additional measurements were also made in supersonic expansion in the region 7–19 GHz. The available data sets for the A and E torsional sublevels were extended to over 1200 transitions, J = 65 and K a = 38 for the ground vibrational state, and to comparable numbers of transitions for first excited states of the methyl torsional mode ν 18, and the in-plane CCN bending mode ν 12. The collected experimental measurements were fitted with several different computer programs for dealing with the effects of methyl torsional motion on the rotational spectrum and many spectroscopic constants have been determined. The experimental results are discussed in detail and are augmented by ab initio computations. Stark effect measurements in supersonic expansion were used to precisely determine the electric dipole moment of pyruvonitrile, ∣ μ a ∣ = 2.462(2) D, ∣ μ b ∣ = 2.442(2) D, μ tot = 3.468(2) D. Pyruvonitrile, as an 8-atom molecule with a sizable dipole moment, is a possible candidate for astrophysical detection and the present work provides the laboratory data necessary for that purpose.

On the Role of Methyl Torsional Modes in the Intersystem Crossing Dynamics of Isolated Molecules

Rotationally resolved fluorescence excitation spectra of the 0(0)(0) bands of the S1<--S0 electronic transitions of 2- and 5-methylpyrimidine (2MP and 5MP, respectively) have been observed and assigned. Both spectra were found to contain two sets of rotational lines, one associated with the sigma=0 torsional level and the other associated with the sigma=+/-1 torsional level of the attached methyl group. Analyses of their structure using the appropriate torsion-rotation Hamiltonian yields the methyl group torsional barriers of V6''=1.56 and V6'=8.28 cm(-1) in 2MP and V6''=4.11 and V6'=58.88 cm(-1) in 5MP. Many of the lines in both spectra are fragmented by couplings with lower lying triplet states. Analyses of some of these perturbations yield approximate values of the intersystem crossing matrix elements, from which it is concluded that the sigma=+/-1 torsional levels of the S1 state are significantly more strongly coupled to the T1 state than the sigma=0 torsional levels.

Rovibronic bands of the Ã←X̃ transition of CH3OO and CD3OO detected with cavity ringdown absorption near 1.2–1.4μm

We have recorded several rovibronic bands of CH3OO and CD3OO in their Ã←X̃ transitions in the range of 1.18–1.40μm 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 ν in the unit cm−1 in this paper) for the COO bending (ν8) and COO symmetric stretching (ν7) modes in the à state are 378 and 887cm−1 for CH3OO, and 348 and 824cm−1 for CD3OO, respectively. In addition, two other vibrational frequencies were observed for the à state of CD3OO, namely, ν5 (954cm−1) and ν6 (971cm−1). These experimental vibrational frequencies for the à 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 ν5 vibrational mode in CD3OO is rationalized by mode mixing with the ν7 mode, as supported by calculations of multidimensional Franck-Condon factors. In addition, many hot bands involving the methyl torsional mode (ν12) 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 X̃ and à states. In addition, the 1222 band in CH3OO and the 1233 band in CD3OO show quite different structures than the origin bands, an effect which results from tunneling splittings comparable to the rotational contour.

The millimeter wave rotational spectrum of pyruvic acid

The rotational spectrum of pyruvic acid has been investigated for the first time in the millimeter-wave region, at 160–314 GHz, and also in supersonic expansion, at 10–17.4 GHz. The analysis of the broadband spectra recorded in this work was carried out with the newly developed AABS software package for Assignment and Analysis of Broadband Spectra, and precise spectroscopic constants are reported for the ground state, the first excited state of the low-frequency skeletal torsional mode ν 24, and the first excited state of the methyl torsional mode ν 23. Limited results have also been obtained for several higher excited states. The dataset for the ground state currently exceeds 1500 lines and for both the A and E internal rotor sublevels spans the complete range of values of K a at the mid values of J for the measured transitions. The results were analysed with three freely available computer programs employing different strategies for dealing with internal rotation and a comparative discussion of their merits is made.

Study of thermodynamic stabilities of polytypes of n-C36H74 by solubility measurements and incoherent inelastic neutron scattering

The thermodynamic properties of the two polytypes of n-hexatriacontane (n-C36H74), single-layered structure Mon and double-layered structure Orth II have been investigated by means of solubility measurements and incoherent inelastic neutron scattering. The solubility measurements reveal that Orth II is more stable than Mon by 1.2 kJ/mol because of the advantage of larger entropy. The neutron scattering measurements show that the vibrational modes of Orth II shift to the lower frequencies compared with those of Mon in the frequency region below 120 cm(-1). The advantage of Orth II in vibrational entropy due to the low-frequency shifts is estimated to be 9.6 J K(-1)/mol at 288 K under the harmonic approximation, which nearly agrees with the entropy difference of 6.8 J K(-1)/mol between Mon and Orth II determined by solubility measurements. These results suggest that the difference in vibrational entropy due to low-frequency modes mainly contributes to the relative thermodynamic stabilities of polytypic structures of long-chain compounds. From the frequency of methyl torsional mode, it is suggested that the cohesive force at the lamellar interface is stronger in Mon than in Orth II.

Internal Methyl Rotation in the CH Stretching Overtone Spectra of Toluene-α-d2, -α-d1, and -d0

The room-temperature vapor phase overtone spectrum of toluene-α-d2 has been recorded in the CH stretching region corresponding to ΔvCH = 2−6 with conventional near-infrared spectroscopy (ΔvCH = 2−4) and with intracavity titanium:sapphire and dye laser photoacoustic spectroscopy (ΔvCH = 4−6). Both absolute oscillator strengths (conventional spectra) and relative oscillator strengths within a given overtone (conventional and photoacoustic spectra) have been measured. The aryl region of the spectrum is interpreted in terms of two nonequivalent aryl local modes and is essentially identical to the aryl regions of the spectra of toluene-d0 and toluene-α-d1. However, the methyl band profile differs significantly in these three molecules. We use an anharmonic oscillator local mode model and an ab initio dipole moment function to calculate oscillator strengths for the aryl and methyl transitions. Parameters for this model come from a fit of the aryl transition energies and from a fit of the methyl spectral profiles. These simple calculations give values that are in good agreement with observed absolute and relative intensities. Differences in the methyl profiles in toluene-d0, -α-d1, and -α-d2 are ascribed to coupling between CH stretching and methyl torsional modes. The methyl profiles are simulated on the basis of a simple adiabatic model that incorporates the harmonically coupled anharmonic oscillator local mode approach and our intensity calculations. The model successfully accounts for the change in methyl profiles between the three molecules, demonstrates the importance of torsional stretching coupling, and shows that coupling between the CH stretching oscillators is unimportant for higher overtones (ΔvCH ≥ 4).

Dynamics of Crystalline N-Methylacetamide: Temperature Dependence of Infrared and Inelastic Neutron Scattering Spectra

N-methylacetamide may be viewed as a simple model for polypeptides. Unusual intensities have previously been observed in its amide vibrational spectra and were attributed to contradictory causes, namely, Fermi resonance, vibrational polarons, and dynamic proton transfer along the H bond. We report the first inelastic neutron scattering study of three specifically deuterated derivatives at different temperatures along with new infrared spectra. The analysis of the librations in the INS spectra indicates a degeneracy between the N−CH3 methyl torsional mode and another degree of freedom assigned to one component of the skeletal torsion. No direct evidence for a methyl−methyl coupling is observed. However γ(CO) appears strongly coupled to the motions of the neighboring C−CH3 group. A new absorption band (“X”) is observed at approximately 1590 cm-1 in the IR spectrum of the amide-1/amide-2 region. This calls into question the previous assignment of a band at ∼1600 cm-1 as the overtone of γ(NH). In addition, all observed peaks in the amide-2 region shift to higher frequency on cooling so that none of them could be assigned to the ν(NH) mode, as previously proposed. Previous theoretical explanations of the anomalous amide spectra of N-methylacetamide based on either Fermi resonance or the proton-transfer model are therefore in disagreement with our new experimental data.

The phosphorescence excitation spectrum of biacetyl: An analysis of the acetyl and methyl torsional mode structure

The T1(n,π*)←S0 laser-induced phosphorescence excitation spectrum of biacetyl has been recorded with a rotating slit nozzle excitation apparatus. The 000 system origin was observed for the first time as a weak band at 19 529 cm−1. Built on this band were the activities of the three large amplitude modes: υ16(au) acetyl torsion, υ15(au) gearing methyl torsion, and υ21(bg), the antigearing methyl torsion. The potential surface for methyl internal rotation extracted from fitting the observed levels yielded a barrier to methyl torsion of 359.6 cm−1.

Ab initio theoretical study of the methyl and phosphine torsion modes in ethylphosphine

In this paper, the far infrared (FIR) methyl and phosphine torsional frequencies and intensities are determined theoretically in ethylphosphine from ab initio calculations. For this purpose, the potential energy function for the double rotation of the methyl and phosphine groups in the electronic ground state is determined in a standard calculation by using the MP2/RHF and a6-31G(3df,p) basis set, with full optimization of the geometry. The numerical results are fitted to a symmetry adapted analytical form and introduced together with the kinetic parameters into the Hamiltonian operator. The Schrödinger equation for these two motions is solved by developing the solutions on the basis of products of trigonometric functions. From the energy levels, the torsional functions and the dipole moment variations the FIR spectrum is synthesized. A new assignment is proposed for some trans-trans and the gauche-gauche transitions between the phosphine levels. Additional transitions between the methyl levels are also proposed. It is concluded that a two dimensional calculation is indispensable for reproducing the FIR torsional spectrum in the region from 200 to 140 cm−1.

Torsion-inversion coupling in the S1 state of acetaldehyde

Coupling between the methyl torsion (υ15) and inversion (υ14) modes is used to model the singlet π*←n spectrum of jet-cooled acetaldehyde and its deuterated analogs. The anomalies observed in the two torsion progressions built onto the lowest inversion doublet are accurately reproduced by using a potential of the form 1/2V3[1−cos3(τ−kq)], where k indicates the proportionality of interaction between the torsion and inversion motions. The coupling also results in intensity to torsion–inversion combination levels that, in the absence of coupling, would not be observed. Consequently, many of the observed transitions are simultaneously Franck–Condon allowed and vibronically induced. The weak origin transition and long progression in the methyl torsion mode is also reproduced in the intensity calculation, indicating that the equilibrium position of the methyl rotor has changed from the S0 eclipsed position to a nearly staggered geometry in the S1 state. The calculated potential energy surface and Franck–Condon intensity calculations predict an S1 equilibrium geometry in which the methyl rotor is staggered with respect to the CO bond by 54° and where the acetyl hydrogen is bent out of the CCO plane by 35°.

Derivation of accurate kinetic and potential-energy functions for several simultaneous large-amplitude vibrations: application to acetaldehyde in the S0and T1states

A procedure is presented for deriving kinetic and potential-energy functions from experimental data for an arbitrary number of large-amplitude vibrations. As a starting point, the method requires a high-quality estimate of the spectroscopic data. We use the previously developed technique of spectrum generation from ab initio calculations to obtain the initial kinetic and potential functions. The present technique minimizes the error between the calculated and experimental energy levels by considering both the kinetic and potential terms. The application to the internal rotation of the methyl group in the S0 state of acetaldehyde shows that the technique is capable of yielding agreement to the experimental measurements to within 0.0001 cm–1. The joint methyl torsion and aldehyde wagging modes for the T1 state of acetaldehyde are also considered. From the two-dimensional potential function the barriers to methyl rotation and to inversion of the aldehydic hydrogen are found to be 633.60 and 935.76 cm–1, respectively. The equilibrium conformation obtained form the two-dimensional potential shows one methyl hydrogen almost eclipsing the aldehydic oxygen (torsion angle 186.40°) whereas the aldehydic hydrogen distorts out of the molecular frame by 39.17°. These structural results are found to differ from that obtained using one-dimensional models, but they agree very well with the results of ab initio calculations.

Spectra and structure of organophosphorus compounds Part LII. Raman and infrared spectra, conformational stability, and ab initio calculations for methoxydichlorophosphine

The Raman (3500 to 30 cm−1) and infrared spectra (3500 to 50 cm−1) of gaseous and solid methoxydichlorophosphine, CH3OPCl2, have been recorded. An infrared study of CH3OPCl2 dissolved in xenon as a function of temperature, as well as a temperature study of the Raman spectra of all phases, definitively shows that only the trans conformer (methyl group trans to the phosphorus lone pair) is present in all phases. The far-infrared spectrum of the gas has been recorded from 350 to 30 cm−1 at a resolution of 0.10 cm−1. The fundamental asymmetric torsional mode has been observed at 120 cm−1, and the frequency of the methyl torsional mode is estimated to be 58 cm−1 from a combination band. A complete vibrational assignment is given for both the normal and d3-isotopomer which is supported by ab initio calculations employing the RHF/3-21G* basis set. The structural parameters and relative conformer stabilities have been obtained with the RHF/6-31G* and MP2/6-31G* basis sets. From these calculations, the gauche conformer is estimated to be more than 3.6 kcal mol−1 less stable than the trans conformer which is consistent with the observation of only the trans conformer in the Raman spectrum of the gas at ambient temperature. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.

A theoretical study of the methyl torsional modes in the [formula omitted]3A u (T 1) ← [formula omitted]1A g (S 0) n → π* spectrum of biacetyl

The S 0→T 1 electronic band spectrum in biacetyl arising from the activity of the methyl torsion modes was simulated by RHF/UHF ab initio calculations. A fit of the calculated band spectrum to the cold jet phosphorescence excitation spectrum provided an assignment of the major bands and predicted that the system origin should lie at 19500 cm −1.