Pulsed field ionization-ZEKE spectroscopy of cresoles and their aqueous complexes: internal rotation of methyl group and intermolecular vibrations

Pulsed field ionization-ZEKE photoelectron spectroscopy and (1 + 1) R2PI spectroscopy have been applied to the cis- and trans-m-cresol.H2O clusters. The internal rotational structure in the S1 state has been re-assigned, and the potential curve has been determined for the cluster. The PFI-ZEKE spectra of the cis- and trans-isomers show low-frequency bands up to 1000 cm-1 above the adiabatic ionization potential IP0. The low-frequency bands are assigned to the internal rotation of the methyl group, the intermolecular stretching and their combination bands in the m-cresol.H2O cluster cation. Level energies and relative transition intensities are reproduced well by a one-dimensional rotor model with a three-fold axis potential. Potential curves for the internal rotation have been determined for both cis- and trans-isomers of m-cresol.H2O cations. The effect of the cluster formation upon the internal methyl rotation, and the interaction between the methyl rotation and the intermolecular vibration are discussed.     

Rotational spectrum of a chiral alpha-hydroxyester: conformation stability and internal rotation barrier heights of methyl lactate

High resolution spectrum of methyl lactate, a chiral alpha-hydroxyester, has been investigated using a molecular jet Fourier transform microwave spectrometer. High level ab initio calculations were employed to study the conformational isomerism of methyl lactate. The observed rotational spectrum confirms that the most stable conformer has an intramolecular hydrogen bond of OH...O==C type, as predicted by the ab initio calculations. The internal rotation barrier heights of the ester methyl group and the alpha-carbon methyl group were calculated to be 5.4 and 14.5 kJ mol(-1) at the MP2/aug-cc-pVDZ level of theory for the most stable conformer. The internal rotation splittings due to the ester methyl group were observed and analyzed and the ester methyl group tunneling barrier height was determined experimentally to be 4.762 (3) kJ mol(-1).     

Shapes and Internal Dynamics of the 1:1 Adducts of Ammonia with trans and gauche Ethanol: A Rotational Study

Two 1:1 adducts of ammonia with ethanol have been characterized by using pulsed‐jet FT microwave spectroscopy. They are formed with two different (trans and gauche), stable conformers of ethanol. Several internal‐dynamics effects are reflected in the features of the rotational spectra. The trans complex shows the tunneling effects owing to internal rotation of both ammonia and the methyl group. The rotational transitions of the gauche species exhibit a small splitting that is related to tunneling through the potential‐energy barrier between the two equivalent minima.     

How trifluoroacetone interacts with water

The rotational spectra of five isotopologues of the molecular adduct 1,1,1-trifluoroacetone-water have been assigned using pulsed-jet Fourier-transform microwave spectroscopy. All rotational transitions appear as doublets, due to the internal rotation of the methyl group. Analysis of the tunneling splittings allows one to determine accurately the height of the 3-fold barrier to internal rotation of the methyl group and its orientation, leading to V(3) = 3.29 kJ·mol(-1) and ∠(a,i) = 67.5°, respectively. The water molecule is linked to the keton molecule on the side of the methyl group through a O-H···O hydrogen bond and a C-H···O intermolecular contact, lying in the effective plane of symmetry of the complex.     

Photophysics and photochemical dynamics of methylanisole molecules in a supersonic jet

The fluorescence excitation, dispersed fluorescence and hole burning spectra, and fluorescence lifetimes of jet-cooled o-, m-, and p-methylanisoles (MA) were measured. The low-frequency ring methyl internal rotational bands observed for their S₀ and S₁ states were assigned. In the case of m-MA, the rotational isomers of cis and trans conformers, which arise from the orientation of the OCH₃ group with respect to the CH₃ group, were assigned by hole-burning spectroscopy. The observed level energies and relative intensities of the methyl internal rotation were reproduced by a calculation using a free rotor basis set. Furthermore, their potentials in the S₀ and the S₁ states were determined. The potential barrier heights for the S₀ states of m- and p-MA were quite low, suggesting that the methyl groups are freely rotating, while changing from S₀ to S₁ states, the potential barrier height increases. The potential barrier heights of o-MA drastically decreased in going from S₀ to S₁ states. The decrease would be due to the hydrogen bonding between O atom and one H atom of the methyl group. The torsional bands of the methoxy group (–OCH₃) were also observed for p- and o-MA. The –OCH₃ modes are found to couple with the level of the e species for the methyl internal rotation.Fluorescence lifetimes (τ_f) of the methyl internal rotational bands in the S₁ states of o-, m-, and p-MA were measured in order to investigate the photochemical dynamics. The values of the nonradiative rate constant (k_nr) were estimated from the τ_f values and Franck–Condon factors. The k_nr values drastically increased with the excitation of methyl internal rotation. Accordingly, the methyl internal rotation should enhance the nonradiative process, presumably intersystem crossing (ISC). The enhancement should be caused by the increase of the state density (ρ) effectively coupled with triplet manifolds. The drastic increase in the ρ value should be caused by level mixing. In addition, the methyl internal rotational motion may enhance the increase of the coupling matrix elements through the vibronic coupling between the excited singlet states. The remarkable rotational quantum species dependence on the ISC rate constant (k_ISC) value clearly appeared in m-MA. The dependence should result from the difference of the ρ value between a₁ and e species, since the e species are doubly degenerate. The species dependence was apparently related to the potential barrier height, suggesting that the large barrier height should have an influence on the ρ value of the triplet states.     

The Structure and Torsional Dynamics of Two Methyl Groups in 2‐Acetyl‐5‐methylfuran as Observed by Microwave Spectroscopy

The molecular‐beam Fourier transform microwave spectrum of 2‐acetyl‐5‐methylfuran is recorded in the frequency range 2–26.5 GHz. Quantum chemical calculations calculate two conformers with trans or cis configuration of the acetyl group, both of which are assigned in the experimental spectrum. All rotational transitions split into quintets due to the internal rotations of two nonequivalent methyl groups. By using the program XIAM, the experimental spectra can be simulated with standard deviations within the measurement accuracy, and yield well‐determined rotational and internal rotation parameters, inter alia the V₃ potentials. Whereas the V₃ barrier height of the ring‐methyl rotor does not change for the two conformers, that of the acetyl‐methyl rotor differs by about 100 cm^?1. The predicted values from quantum chemistry are only on the correct order of magnitude.     

Internal dynamics features in the free jet rotational spectrum of the acetaldehyde-Kr molecular complex

The structure and the dynamics of internal motions in the complex formed between acetaldehyde and Kr are studied by free jet absorption microwave spectroscopy performed in the range 60-78 GHz. The fourfold structure of each rotational line is evidence of the vibration-rotation coupling between the overall rotation of the complex, a tunneling motion of the Kr atom between two equivalent positions and the internal rotation of the methyl group in the acetaldehyde moiety. The four sets of transitions could be fitted with a coupled Hamiltonian which allows for the Coriolis interaction obtaining the energy separation between the vibrational energy levels related to the tunneling motion, while the observed splittings due to the methyl group internal rotation were analyzed independently with an appropriate model. The potential energy barriers for the tunneling motion and the internal rotation of the methyl group have been calculated and the interaction of the rare gas atom with the acetaldehyde moiety is reflected in the change of the V(3) barrier to internal rotation in going from the molecule to the weakly bound complex.     

Impact of molecular conformation on barriers to internal methyl rotation: the rotational spectrum of m-methylbenzaldehyde

The ground state spectrum of m-methylbenzaldehyde (m-MBA) was measured with a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. The methyl rotor on m-MBA introduces an internal rotation barrier, which leads to splitting of the torsional energy level degeneracy into A and E states. Ab initio calculations predict a low torsional barrier for both the O-cis and O-trans conformers, resulting in a large doublet splitting up to several gigahertz in the frequency spectrum. The rotational constants, distortion terms, and V(3) values for both species have been determined from the ground state rotational spectrum using the BELGI-C(s) fitting program. There are significant differences in the torsional potential for the O-cis and O-trans m-MBA conformers. Molecular orbitals and resonance structures for each conformer are analyzed to understand the difference in torsional barrier height as well as the irregular shape of the O-trans torsional potential.     

Conformational Equilibria and Large‐Amplitude Motions in Dimers of Carboxylic Acids: Rotational Spectrum of Acetic Acid–Difluoroacetic Acid

We report the rotational spectra of two conformers of the acetic acid–difluoroacetic acid adduct (CH₃COOH–CHF₂COOH) and supply information on its internal dynamics. The two conformers differ from each other, depending on the trans or gauche orientation of the terminal ?CHF₂ group. Both conformers display splittings of the rotational transitions, due to the internal rotation of the methyl group of acetic acid. The corresponding barriers are determined to be V₃(trans)=99.8(3) and V₃(gauche)=90.5(9) cm^?1 (where V₃ is the methyl rotation barrier height). The gauche form displays a further doubling of the rotational transitions, due to the tunneling motion of the ?CHF₂ group between its two equivalent conformations. The corresponding B₂ barrier is estimated to be 108(2) cm^?1. The increase in the distance between the two monomers upon OH→OD deuteration (the Ubbelohde effect) is determined.     

Internal rotational motion of the chloromethyl group of the jet-cooled benzyl chloride molecule

The mass-resolved resonance enhanced two-photon ionization spectra of jet-cooled benzyl chloride were measured. Some low-frequency vibronic bands around the S1-S0 origin band were assigned to transitions of the internal rotational mode of the chloromethyl group. The internal rotational motion was analyzed by using the one-dimensional free rotor approximation. The conformation in the S1 state was found to be that in which the C-Cl bond lies in orthogonal to the benzene plane. For the species with m/e 126, the transition energy of the internal rotational bands corresponded well to the potential energy values of V2 = 1900 cm(-1) and V4 = 30 cm(-1) in the S1 state and the reduced rotational constant B values 0.50 and 0.47 cm(-1) in the S0 and S1 states, respectively. The B values obtained for the chlorine isotopomer (m/e 128) were slightly different. The S1 potential barrier height was found to be about 3 times larger than that for the S0 state. Molecular orbital calculations suggest that the difference between energies of the HOMO and LUMO with respect to the rotation of the chloromethyl group correspond approximately to the potential energy curve obtained for the S1 state.     

Large Amplitude Torsions in Nitrotoluene Isomers Studied by Rotational Spectroscopy and Quantum Chemistry Calculations

Rotational spectra of ortho-nitrotoluene (2-NT) and para-nitrotoluene (4-NT) have been recorded at low and room temperatures using a supersonic jet Fourier Transform microwave (MW) spectrometer and a millimeter-wave frequency multiplier chain, respectively. Supported by quantum chemistry calculations, the spectral analysis of pure rotation lines in the vibrational ground state has allowed to characterise the rotational energy, the hyperfine structure due to the ¹⁴N nucleus and the internal rotation splittings arising from the methyl group. For 2-NT, an anisotropic internal rotation of coupled −CH₃ and −NO₂ torsional motions was identified by quantum chemistry calculations and discussed from the results of the MW analysis. The study of the internal rotation splittings in the spectra of three NT isomers allowed to characterise the internal rotation potentials of the methyl group and to compare them with other mono-substituted toluene derivatives in order to study the isomeric influence on the internal rotation barrier.     

HD isotope effect of methyl internal rotation for acetaldehyde in ground state as calculated from a multicomponent molecular orbital method

We have analyzed the differences in the methyl internal rotation induced by the HD isotope effect for acetaldehyde (CH(3)CHO) and deuterated acetaldehyde (CD(3)CDO) in ground state by means of the multicomponent molecular orbital (MC_MO) method, which directly accounts for the quantum effects of protons and deuterons. The rotational constant of CH(3)CHO was in reasonable agreement with experimental one due to the adequate treatment of the protonic quantum effect by the MC_MO method. The C-D bond distances were about 0.007 A shorter than the C-H distances because of the effect of anharmonicity of the potential. The Mulliken population for CD(3) in CD(3)CDO is larger than that for CH(3) in CH(3)CHO because the distribution of wavefunctions for the deuterons was more localized than that for the protons. The barrier height obtained by the MC_MO method for CH(3)CHO was estimated as 401.4 cm(-1), which was in excellent agreement with the experimentally determined barrier height. We predicted the barrier height of CD(3)CDO as 392.5 cm(-1). We suggest that the internal rotation of the CD(3) group was more facile than that of the CH(3) group because the C-D bond distance was observed to be shorter than the C-H distance. Additionally the localized electrons surrounding the CD(3) group in CD(3)CDO caused the extent of hyperconjugation between the CD(3) and CDO groups to be smaller than that in the case of CH(3)CHO, which may have also contributed to the observed differences in methyl internal rotation. The differences in bond distances and electronic populations induced by the H/D isotope effect were controlled by the difference in the distribution of wavefunctions between the protons and deuterons.     

Partitioning of methyl internal rotational barrier energy of thioacetaldehyde

The nature of methyl internal rotational barrier in thioacetaldehyde has been investigated by relaxation effect, natural bond orbital (NBO) analysis and Pauling exchange interactions. The true experimental barrier can be obtained by considering fully relaxed rotation. Nuclear-electron attraction term is a barrier forming term in the fully relaxed rotation, but it appears as an antibarrier for rigid rotation. It is seen that during methyl rotation, the torsional mode is coupled with the aldehydic hydrogen out-of-plane wagging motion. Natural bond orbital analysis shows that the principal barrier forming term originates from the C-C bond. The lengthening of the C-C bond is explained by considering charge transfer interaction between several bonding and antibonding orbitals in the C-C bond region, which leads to higher bonding overlap for the eclipsed conformer compared to the staggered conformer. S-C(σ)/C^me-Hp and C-H_ald/C_me-H_op interactions appear to be the main barrier-forming Pauling exchange terms but have less contribution to make to the barrier compared to the C-C bond interaction.     

Comments on steric effects in buttressed methyl pyridine species

Using MINDO/3 calculations, we have investigated the importance of gear effects and pointing H-H interactions in several polymethylpyridine systems. Gear clashing of methyl groups on adjacent ring positions was found to produce about 0.6 kcal mole^-1 stabilization, while interaction between two pointing hydrogens destabilizes the molecule by about 1.1 kcal mo1e^-1. Calculated barriers for rotation of the isopropyl group in several 1-isopropyl-poly methyl pyridiniun cations were in good agreement with experimental results published recently by Roussel. The calculations indicate that the decrease in the rotational barrier upon additional buttressing of methyl groups can be attributed to destabilization of the ground rotational conformer for the more sterically hindered 1-isopropyl polymethyl pyridinium species.     

On the internal rotations in p-cresol in its ground and first electronically excited states

The overall rotation and internal rotation of p-cresol (4-methyl-phenol) has been studied by comparison of the microwave spectrum with accurate ab initio calculations using the principal axis method in the electronic ground state. Both internal rotations, the torsions of the methyl and the hydroxyl groups relative to the aromatic ring, have been investigated. The internal rotation of the hydroxyl group can be approximately described as the motion of a symmetrical rotor on an asymmetric frame. For the methyl group it has been found that the potential barrier hindering its internal rotation is very small with the first two nonvanishing Fourier coefficients of the potential V(3) and V(6) in the same order of magnitude. Different splittings of b-type transitions for the A and E species of the methyl torsion indicate a top-top interaction between both internal rotors through the benzene ring. An effective coupling potential for the top-top interaction could be estimated. The hindering barriers of the hydroxyl and methyl rotation have been calculated using second-order Moller-Plesset perturbation theory and the approximate coupled-cluster singles-and-doubles model (CC2) in the ground state and using CC2 and the algebraic diagrammatic construction through second order in the first electronically excited state. The results are in excellent agreement with the experimental values.     

13C NMR relaxation study of molecular motions in tetraphenyltin and tetra(p-tolyl)tin in solution

The Woessner approach is applied to the 13C relaxation data for tetraphenyltin (1) and tetra(p-tolyl)tin (2) in CDCl3 solution over the temperature range 5-42 degrees C to obtain correlation times for rotational motions and hence the activation barriers. Quantum mechanical computations were carried out to obtain the rotational energy barriers for comparison. For 2 the relaxation data indicate (1) slower ring rotation than in 1, (2) highly hindered internal rotation of the methyl group. IR and chemical shift data support the hypothesis of hyperconjugation of the methyl correlated with interaction between the pi-electrons and the 5d orbitals of tin in the (p-tolyl)Sn moiety to account for the hindrances to the rotations of the ring and the methyl. The activation barrier for the tolyl group rotation is found to be much higher than that for the phenyl rotation. However, the Woessner approach yields an anomalously high barrier for the methyl rotation. An explanation based on correlated rotations of the tolyl ring and the methyl is offered.     

Internal Rotation of the Acetyl Methyl Group in Methyl Alkyl Ketones: The Microwave Spectrum of Octan-2-one

Methyl n-alkyl ketones form a class of molecules with interesting internal dynamics in the gas-phase. They contain two methyl groups undergoing internal rotations, the acetyl methyl group and the methyl group at the end of the alkyl chain. The torsional barrier of the acetyl methyl group is of special importance, since it allows for the discrimination of the conformational structures. As part of the series, the microwave spectrum of octan-2-one was recorded in the frequency range from 2 to 40 GHz, revealing two conformers, one with C₁ and one with C_s symmetry. The barriers to internal rotation of the acetyl methyl group were determined to be 233.340(28) cm⁻¹ and 185.3490(81) cm⁻¹, respectively, confirming the link between conformations and barrier heights already established for other methyl alkyl ketones. Extensive comparisons to molecules in the literature were carried out, and a small overview of general trends and rules concerning the acetyl methyl torsion is given. For the hexyl methyl group, the barrier height is 973.17(60) cm⁻¹ for the C₁ conformer and 979.62(69) cm⁻¹ for the C_s conformer.     

The C(2v) structure of enolic acetylacetone

It has often been postulated that the lowest energy enolic form of Acetylacetone (AcAc) assumes C(s) symmetry, i.e., has a double-minimum potential possibly exhibiting a low barrier to internal proton transfer and not a single minimum, C(2v). Recent theoretical calculations and experimental work support the C(s) hypothesis but the literature on this fascinating molecule is divided. Toward this objective, the high-resolution rotational spectra of enolic acetylacetone and 3 isotopologues have been obtained, revealing C(2v) symmetry. The two methyl groups exhibit a very low barrier to internal rotation, thus making AcAc internally highly dynamic.     

Microwave spectra and barriers to internal rotation of z- and e-1-propenyl isocyanide

A synthetic procedure yielding a mixture of Z- and E-1-propenyl isocyanide (CH(3)CH═CHNC) is described. The microwave spectrum of this mixture has been recorded in the 12-100 GHz spectral range, and the spectra of the Z and E isomers have been assigned for the first time. Most transitions of the Z form were split into two components of equal intensity due to tunneling of the methyl group, which allowed the barrier to internal rotation of this group to be determined as 4.0124(12) kJ/mol by fitting 568 transitions with a maximum value of J = 46 using the computer program Xiam. This fit had a root-mean-square deviation as large as 4.325. The same transitions were therefore fitted anew using the more sophisticated program Erham. This fit has a rms deviation marginally better (4.136) than the Xiam fit. No split MW lines were found for E-1-propenyl isocyanide. The absence of splittings is ascribed to a barrier to internal rotation of the methyl group that is significantly higher than the barrier of the Z isomer. It is concluded that the barrier must be larger than 6 kJ/mol for the E form. The experimental work was augmented by quantum chemical calculations at CCSD/cc-pVTZ, B3LYP/cc-pVTZ, and MP2/cc-pVTZ levels of theory. The CCSD method predicts rotational constants of the Z and E forms well. The B3LYP barriers to internal rotation of a series of substituted propenes were calculated and found to be in good agreement with experiments. Calculations of the quartic centrifugal distortion constants of the two 1-propenyl isocyanides by the B3LYP and MP2 methods were less successful.