Barrier Height For Internal Rotation Research Articles

Revealing Internal Rotation and 14N Nuclear Quadrupole Coupling in the Atmospheric Pollutant 4-Methyl-2-nitrophenol: Interplay of Microwave Spectroscopy and Quantum Chemical Calculations.

The structure and interactions of oxygenated aromatic molecules are of atmospheric interest due to their toxicity and as precursors of aerosols. Here, we present the analysis of 4-methyl-2-nitrophenol (4MNP) using chirped pulse and Fabry-Pérot Fourier transform microwave spectroscopy in combination with quantum chemical calculations. The rotational, centrifugal distortion, and 14N nuclear quadrupole coupling constants of the lowest-energy conformer of 4MNP were determined as well as the barrier to methyl internal rotation. The latter has a value of 106.4456(8) cm-1, significantly larger than those from related molecules with only one hydroxyl or nitro substituent in the same para or meta positions, respectively, as 4MNP. Our results serve as a basis to understand the interactions of 4MNP with atmospheric molecules and the influence of the electronic environment on methyl internal rotation barrier heights.

Electronic excitation spectra of jet-cooled phenyl isocyanate, m-tolyl isocyanate, and p-phenylene diisocyanate: Assignment of the cis and trans rotational isomers.

To assign cis and trans isomers of m-tolyl isocyanate (mTI) and p-phenylene diisocyanate (pPDI) in the electronic excitation transition, we measured the time-of-flight mass-selected resonant ionization spectra of jet-cooled phenyl isocyanate (PI), mTI, and pPDI in the region of the 275nm first ππ* absorption system. In the excitation spectra of jet-cooled mTI and pPDI, cis- and trans-rotational isomers appeared as doublets. Isomers were assigned by analyzing the methyl-group internal rotation for mTI and by applying low-frequency bending vibrations to the mutual exclusion rule between the one- and two-photon spectra for pPDI. The electronic spectra of the three molecules observed in the jet were assigned to the transition to the first ππ* and third singlet excited states with the aid of time-dependent (TD)-B3LYP/aug-cc-pVDZ and TD-CAM-B3LYP/aug-cc-pVDZ calculations. The 0 - 0 band of PI was observed at 36 354cm-1, those of the cis and trans isomers of mTI at 36 018 and 35 853cm-1, respectively, and those of the cis and trans isomers of pPDI at 34 437 and 34 383cm-1, respectively. All vibronic bands were diffuse, probably because of internal conversion to two singlet nπ* states. For mTI, based on changes in the barrier height of methyl-group internal rotation upon excitation, the Hammett-σm of PI was determined to be -0.12.

Conformational landscape and internal dynamics of limona ketone, a key oxidation product of limonene

The rotational spectrum of limona ketone (4-acetyl-1-methyl-1-cyclohexene), a key oxidation product of limonene, was recorded in the gas phase using a Fourier transform microwave spectrometer coupled to a supersonic jet expansion over the 4–20 GHz range. To display a complete view of the conformational landscape of this relatively flexible system, the relative stability and inter-conversion barriers between conformers were explored by electronic structure calculations. The lowest energy equatorial conformer was detected, and its spectroscopic molecular parameters were determined from the analysis of the spectrum. The internal rotation of the methyl group attached to the aldehyde moiety was taken into account in the analysis. The internal dynamics of the methyl group within the acetyl moiety was quantitatively described. Experimental lines of both A and E symmetry were fitted to experimental accuracy, and the experimental barrier height of the methyl group internal rotation is discussed with the support of quantum chemical calculations.

Water binding to the atmospheric oxidation product methyl vinyl ketone

Methyl vinyl ketone is one of the major oxidation products of isoprene, and therefore, an important precursor of secondary organic aerosol. Understanding its interactions with water is relevant to gain insight into aerosol formation and improve the predictive power of atmospheric chemistry models. The molecular complex formed between methyl vinyl ketone and water has been generated in a supersonic jet and characterized using high-resolution microwave spectroscopy in combination with quantum chemistry calculations. In this study, we show that methyl vinyl ketone interacts with water forming four 1:1 isomers connected by O − H···O and C − H···O hydrogen bond interactions. Water has been found to preferentially bind to the antiperiplanar conformation of methyl vinyl ketone. Evidence of a large amplitude motion arising from the methyl internal rotation has been found in the rotational spectra of the dimer. The threefold methyl internal rotation barrier heights have been further determined and discussed for all the species.

The millimeter-wave spectrum and astronomical search for ethyl methyl sulfide.

Sulfur-containing molecules constitute only 8% of the molecules observed in the interstellar medium (ISM), in spite of the fact that sulfur has been shown to be an abundant element in the ISM. In order to understand the chemical behavior of the ISM and specific cases like the missing sulfur reservoir, a detailed chemical molecular composition in the ISM must be mapped out. Our goal is to investigate the rotational spectrum of ethyl methyl sulfide, CH3CH2SCH3, which ms to be a potential candidate for observation in the ISM since the simpler analogs, CH3SH and CH3CH2SH, have already been detected. Rotational spectrum of ethyl methyl sulfide has been observed before, but its experimental rotational parameters are not precise enough to allow its detection in the ISM. The rotational spectrum of ethyl methyl sulfide in the frequency range 72-116.5 GHz was measured using a broadband millimeter-wave spectrometer based on radio astronomy receivers with fast Fourier transform backends. The spectral searches and identification of the vibrational excited states of ethyl methyl sulfide was supported by high-level ab initio calculations on the harmonic and anharmonic force fields. The rotational spectra for the trans and gauche conformers of ethyl methyl sulfide was analyzed, and a total of 172 and 259 rotational transitions were observed for each one, respectively. The observation of A - E internal rotation splittings allowed the experimental determination of the V 3 hindered internal rotation barrier height for both trans and gauche species. In addition, the vibrational excited states, resulting from the lowest frequency vibrational mode ν 30 were identified for both conformers. The new experimental rotational parameters were employed to search for ethyl methyl sulfide in the warm and cold molecular clouds Orion KL, Sgr B2(N), B1-b and TMC-1, using the spectral surveys captured by IRAM 30 m at 3 mm and 2 mm.

Structure and internal rotation dynamics of the acetone-neon complex studied by microwave spectroscopy

The microwave spectra of the van der Waals complexes acetone-20Ne and acetone-22Ne were measured using a cavity-based supersonic jet Fourier-transform microwave spectrometer in the region from 5 to 18GHz. For these two isotopologues, both c- and weaker a-type transitions were observed. The transitions are split into multiplets due to the internal rotation of the two methyl groups in acetone. Initial electronic structure calculations were performed at the MP2/6-311++g (2d,p) level of theory and the internal rotation barrier height of the methyl groups was calculated to be ∼2.8kJ/mol. The ab initio rotational constants were the basis for the spectroscopic searches, but the multiplet structures and floppiness of the complex made the quantum number assignment very difficult. The assignment was finally achieved with the aid of constructing closed frequency loops and predicting internal rotation splittings using the XIAM internal rotation program. The acetone methyl group tunneling barrier height was determined experimentally to be 3.10(6)kJmol−1 [259(5)cm−1] in the acetone-Ne complex, which is lower than in the acetone monomer but comparable to the acetone-Ar complex (Kang et al., 2002). Experimental data and high-level CCSD(T)/aug-cc-pVTZ calculations suggest that the Ne atom lies directly above the plane formed by the carbonyl group and the two carbon-carbon bonds, which is different than the slightly offset position found previously in the acetone-Ar complex. Additionally, ab initio calculations and Quantum Theory of Atoms in Molecules analyses were used to analyze the methyl internal rotation motions in acetone and acetone-Ne.

Internal rotation and equilibrium structure of 2-chloro-3-nitrothiophene from gas electron diffraction and quantum chemistry

The equilibrium structure of the 2-chloro-3-nitrothiophene molecule and the internal rotation of the nitro group have been studied in gas phase using electron diffraction data and quantum chemical calculations in the framework of the large-amplitude motion model for internal rotation.Quantum chemical calculations at the MP2 and B3LYP levels of theory with various Pople and Dunning basis sets predict the planar minimum energy molecular conformation, with the internal rotation barrier height in the interval 1140–1560 cm−1.The experimental GED data are consistent with the dynamic model governed by the twofold cosine potential energy function with the barrier height in the range from 600 to 1400 cm−1.The main equilibrium structure parameters are as follows (values in parentheses correspond to 3 times standard deviations): re (NO) = 1.225/1.227(3) Å, re(C − C)/re(C − N) = 1.362/1.376/1.403/1.438(3) Å, re(C − Cl)/re (C − S) = 1.700/1.712/1.715(2) Å, ∠e C2SC5 = 92.2 (5)°, ∠e C3C2S = 110.8 (6)°, ∠e C4C5S = 111.3 (5)°, ∠e C3C4C5 = 113.0 (7)°, ∠e C2C3C4 = 112.7 (8)°, ∠e C3NO11 = 116.9(6)°, ∠eC3NO10 = 118.5 (6)°, ∠e SCCl = 118.6 (5)°, ∠e C4C3N = 121.8 (7)°, ∠e ONO = 124.6(6)°, ∠e C2C3N = 125.1 (6)°, ∠e C3C2Cl = 130.7 (6)°.Thermally averaged parameters are provided for comparison with the results of traditional studies.

Electron diffraction analysis for the molecules with multiple large-amplitude motions. 3-nitrostyrene--a molecule with two internal rotors.

Dynamic structural analysis of the molecules possessing large-amplitude degrees of freedom has been attempted by many researchers; however, so far, electron diffraction investigations involved only one large-amplitude coordinate (internal rotation or bending). The current state of computational facilities allows extending of the general dynamic approach to the systems possessing two or more large-amplitude motions. This paper presents the first practical implementation of the theoretical method developed previously by the authors for solving the dynamic-structural problem with two or more large-amplitude coordinates; the procedure is applied to a molecule of 3-nitrostyrene. The molecule is represented as a set of pseudoconformers built on a two-dimensional grid corresponding to both internal rotation coordinates present in the molecule (with 10-30° steps by each angle); altogether, up to 342 pseudoconformers were used. Structural analysis was based on the experimental electron diffraction data supported by quantum chemical calculations (at the MP2 and B3LYP levels of theory) and molecular spectroscopy data. Quantum chemistry predicts the planar structure of both syn- and anti- stable conformations with close energies and weak interaction between internal rotations of nitro and vinyl groups. The gas-phase electron diffraction experimental data are compatible with the quantum chemical predictions. The principal equilibrium geometry parameters of the molecule (syn- conformation) have been determined as follows: r(e)(C-C)(ring, avg.) = 1.391(1) Å, r(e)(C-C) = 1.477(5) Å, r(e)(C═C) = 1.333(7) Å, r(e)(C-N) = 1.463(5) Å, r(e)(N═O) = 1.227(3) Å, ∠e(O═N═O) = 124.3 (4)°. Experimental data for this molecule are insufficient to make estimates of the barrier heights of internal rotation; the population ratio of syn- and anti- conformations is evaluated as 50 ± 20%. Results of our investigation confirm the presence of significant internal rotations in the 3-nitrostyrene molecule.

Chirped-Pulse and Cavity-Based Fourier Transform Microwave Spectroscopy of a Chiral Epoxy Ester: Methyl Glycidate

Rotational spectra of a chiral epoxy ester, methyl glycidate, were measured using a chirped-pulse and a cavity-based Fourier transform microwave spectrometer. The two lowest energy conformers where the epoxy oxygen and the ester oxygen atoms are in the syn and anti relative orientation with respect to each other were identified experimentally. Spectra of four (13)C isotopologues of the lowest energy conformer of methyl glycidate were also measured and assigned. All of the observed rotational transitions are split into doublets due to the presence of the ester methyl internal rotor. The rotational constants and the internal rotation barrier height for the ester methyl group were determined for both conformers of methyl glycidate and for the four (13)C isotopologues of the most stable conformer. A value for the interconversion barrier between the two most stable conformers was estimated. Furthermore, comparison to strawberry aldehyde, a larger derivative of methyl glycidate, shows how the syn-anti conformational equilibrium shifts as a result of the additional bulky substituents at the epoxy ring and at the ester oxygen atom.

Hydrogen bond and internal rotations barrier: DFT study on heavier group-14 analogues of formamide

A theoretical study on heavier group-14 substituting effect on the essential property of formamide, strong hydrogen bond with water and internal rotational barrier was performed within the framework of natural bond orbital (NBO) analysis and based on the density functional theory calculation. For heavier group-14 analogues of formamide (YHONH2, Y = Si, Ge and Sn), the nN–πY=O conjugation strength does not always reduce as Y becomes heavier, for example, silaformamide and germaformamide have similar strength of delocalization. Heavier formamides prefer being H-bond donors to form FYO–H2O complexes to being H-bond acceptors to form FYH–H2O complexes. The NEDA analysis indicates that H-bond energies of FYO–H2O complexes increase as moving down group 14 due to concurrently stronger charge transfer (CT) and electrostatic attraction and for the FYH–H2O complexes H-bond strengths are similar. The model of CTs from FYO to H2O differs from that at FYH–H2O complexes, which are contributed not only by aligning lone-pair orbital of O but also by another lone-pair orbital. At two lowest lying excited states (the triplet and S1 excited states), formamide and its heavier analogues form double H-bonds with H2O molecule at the same time. The barrier heights of internal rotation become gradually low from C to Sn, formamide (15.73 kcal/mol) > silaformamide (11.73 kcal/mol) > germaformamide (9.45 kcal/mol) > stannaformamide (7.50 kcal/mol) at the CCSD(T)/aug-cc-pVTZ//B3LYP/cc-pVTZ level. NBO analysis indicates that the barrier does not only come from the nNπ*YO conjugation, and for heavier analogues of formamide, the nNσ*YO hyperconjugation effect and steric effect considerably contribute to the overall rotational barrier. Copyright © 2013 John Wiley & Sons, Ltd.

ChemInform Abstract: Rotational Spectra of p-Isopropylbenzaldehyde: Assignment of Two Conformers and Observation of Torsionally Excited States.

Abstract Rotational spectra of p -isopropylbenzaldehyde were obtained at a very low temperature in a pulsed-beam Fourier transform-microwave spectrometer and over a range of much higher temperatures in a continuous wave spectrometer. Two species with ground-state rotational constants A =2975.15(14) MHz, B =550.1631(1) MHz, C =516.3000(1) MHz and A =3183.80(30) MHz, B =538.8124(1) MHz, C =512.0049(1) MHz were observed in the beam and are consistent with structures having the methine hydrogen coplanar with the benzene ring and respectively anti and syn to the carbonyl oxygen of the para formyl group. Relative intensities of the rotational transitions indicate that the two conformers have almost equal energies. Near room temperature, in addition to R-branch a -type band series consistent with the anti and syn conformers, a very intense band series with an intermediate B + C value is observed in the low-resolution microwave spectrum of p -isopropylbenzaldehyde, as previously reported [N.S. True, J. Phys. Chem., 87 (1983) 4622]. Temperature-dependent relative intensity measurements do not support the proposal of these authors that the intensity of this series reflects the population in torsional states above the torsional barrier and can be used to estimate the internal rotation barrier height. Additional factors not directly related to the free rotor population must contribute to the observed intensity.

Cis– trans isomerization of carbon double bonds in monounsaturated triacylglycerols via generation of free radicals

We investigated the heat-induced cis/ trans isomerization of double bonds in monounsaturated lipids. When triolein (9- cis, 18:1) was heated around 180 °C, small amounts of isomerization products were obtained depending on the heating period. The heat-induced isomerization of triolein was considerably suppressed by the addition of different antioxidants or under nitrogen stream, and these additives simultaneously inhibited the thermal oxidation of double bonds in triolein. Therefore, an intermediate of the thermal oxidation reaction might be responsible for the heat-induced isomerization of the double bonds in triolein. The thermodynamics of the heat-induced isomerization of triolein (9- cis, 18:1) and trielaidin (9- trans, 18:1) were investigated using Arrhenius plot. The Arrhenius activation energies of cis double bonds in triolein and trans double bonds in trielaidin were 106 kJ/mol and 137 kJ/mol, respectively. The calculated internal rotational barrier heights of these double bonds were similar to those of the double bond of 2-butene radical and significantly lower than those of non-radicalized double bonds in 2-butene. These results suggest that heat-induced cis/ trans isomerization of triolein and trielaidin occurs mainly through the formation of radical species, which are the intermediates produced during thermal oxidation. The activation energy difference between the two forms suggests that trans trielaidin radicals are more stable than cis triolein radicals. The high thermodynamic stability of the trans double bonds in lipid radicals would influence the population of cis and trans isomers in edible oils and contribute to slight accumulation of trans-18:1 isomers during heating or industrial processing.

Calculation of intensities of torsional-rotational bands in the IR absorption spectrum of hydrogen peroxide

We present the calculated intensity distributions in torsional-rotational IR absorption bands of hydrogen peroxide. The torsional components of the band intensities have been calculated based on the appropriate matrix element computations. The contribution of the rotational components has been calculated using the 3j-symbols technique. The calculations have proved the reliability of available data on rotational constants, barrier heights of internal rotation, and locations of torsional-rotational levels of hydrogen peroxide.

Reanalysis of the microwave absorption spectrum of dimethyl methylphosphonate and its internal rotation problem

The Fourier transform microwave spectrum of dimethyl methylphosphonate studied by us previously is reanalyzed here to obtain more physically reasonable parameters describing the various Coriolis-like couplings between overall rotation and internal rotation of the two methoxy methyl tops. In particular, we use exactly the same frequencies and spectral assignments as in our previous study, but the least squares fit is started from a rather different set of initial molecular parameters and is carried out with a slightly smaller set of adjustable parameters. The standard deviation of the fit is not significantly changed, but convergence to a rather different minimum in parameter space is obtained. This new minimum does not change the three rotational constants significantly, but values for the 12 Coriolis coupling constants are dramatically rearranged, so that parameters arising from coupling between the two internal rotation motions are greatly reduced in magnitude. These new Coriolis constants bring the derived direction cosines for the methoxy methyl groups in the principal axis system into much better agreement with ab initio predictions. We have used our new parameters to derive internal rotation barrier heights for the two methyl groups of 280 and 188 cm −1.

Rotational spectrum of a chiral α-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).

Statistical thermodynamics of bond torsional modes: Tests of separable, almost-separable, and improved Pitzer–Gwinn approximations

Practical approximation schemes for calculating partition functions of torsional modes are tested against accurate quantum mechanical results for H(2)O(2) and six isotopically substituted hydrogen peroxides. The schemes are classified on the basis of the type and amount of information that is required. First, approximate one-dimensional hindered-rotator partition functions are benchmarked against exact one-dimensional torsion results obtained by eigenvalue summation. The approximate one-dimensional methods tested in this stage include schemes that only require the equilibrium geometries and frequencies, schemes that also require the barrier heights of internal rotation, and schemes that require the whole one-dimensional torsional potential. Then, three classes of approximate full-dimensional vibrational-rotational partition functions are calculated and are compared with the accurate full-dimensional path integral partition functions. These three classes are (1) separable approximations combining harmonic oscillator-rigid rotator models with the one-dimensional torsion schemes, (2) almost-separable approximations in which the nonseparable zero-point energy is used to correct the separable approximations, and (3) improved nonseparable Pitzer-Gwinn-type methods in which approaches of type 1 are used as reference methods in the Pitzer-Gwinn approach. The effectiveness of these methods for the calculation of isotope effects is also studied. Based on the results of these studies, the best schemes of each type are recommended for further use on systems where a corresponding amount of information is available.

Torsional potential of 4,4′-bipyridine: Ab initio analysis of dispersion and vibrational effects

Ab initio calculations using restricted Hartree-Fock, second-order Møller-Plesset perturbation theory (MP2), density-functional theory (DFT), and coupled-cluster methods have been done to obtain the torsional potential-energy profile of the aza-aromatic molecule 4,4'-bipyridine. The torsional potential is evaluated adiabatically by fixing the normalized sum of the dihedral angles through the C-C inter-ring bond at several values along the torsional path and relaxing the remaining degrees of freedom. Previous discrepancies between MP2 and DFT internal rotation barrier heights are removed, and seen to be mostly due to the underestimation of the dispersion energy in the coplanar conformer by MP2 when using relatively small basis sets. The calculations indicate that the barrier height between the twisted global minimum and the 0 degrees conformer is around 1.5-1.8 kcal mol-1 while that corresponding to the 90 degrees one is about 2.0-2.2 kcal mol-1. This same relative energy ordering of the coplanar and perpendicular conformers was experimentally derived from nuclear magnetic resonance (NMR) measurements of 1H dipolar couplings on 4,4'-bipyridine solutions in a nematic liquid crystal, although the barrier heights are much lower than those estimated from NMR experiments in the gas phase. The DFT infrared spectrum and zero-point vibrational energy corrections to the torsional energy profile have also been calculated, the latter having a small influence on the torsional potential-energy profiles.

Effect of the Methyl Internal Rotation Barrier Height on CH−Stretching Overtone Spectra

The coupling of CH stretching with methyl internal rotation (torsion) causes complexity in methyl CH stretching overtone spectra. We have used a CH stretching methyl torsion model to simulate the methyl band overtone profiles. Room-temperature vapor phase overtone spectra of acetic acid (ΔvCH = 4−5) and 2-methylfuran (ΔvCH = 4−5) have been recorded in the methyl CH stretching regions. These combined with previously studied molecules acetone, acetaldehyde, toluene, methylpyridines, and xylenes provide a series of molecules with gradually varying barrier heights.

High-Level ab Initio Calculations of Torsional Potential of Phenol, Anisole, and o-Hydroxyanisole: Effects of Intramolecular Hydrogen Bond

The internal rotational barrier heights of phenol and anisole were calculated using several basis sets up to cc-pVQZ with MP2-level electron correlation correction to evaluate the basis set effects. The calculations showed that the effects of the further improvement of the basis set beyond the cc-pVTZ were very small. Although the electron correlation substantially increased the barrier heights of the two molecules, the effects of the electron correlation beyond the MP2 method were not large. The barrier heights calculated with the CCSD(T) method were close to those with the MP2 method. The internal rotational potentials of methoxy and hydroxyl groups of o-hydroxyanisole were calculated at the MP2/cc-pVTZ//HF/6-311G** level. The calculated potentials were compared with those of phenol and anisole. o-Hydroxyanisole preferred planar structure in which the hydroxyl group had an intramolecular hydrogen bond with the oxygen atom of the methoxy group. The calculated torsional potential of the methoxy group had the maximum (7.30 kcal/mol) when the methoxy group rotated 180° from the minimum energy structure, in which the hydroxyl group did not have the hydrogen bond. The barrier height of the methoxy group of o-hydroxyanisole was considerably larger than that of anisole (2.99 kcal/mol). The large internal rotational barrier height of o-hydroxyanisole showed that the intramolecular hydrogen bond greatly stabilized the energy minimum structure and that the hydrogen bond strictly restricted the conformational flexibility of the methoxy group.

The Rotational Spectrum of thep-Fluorotoluene– Argon van der Waals Complex

The microwave rotational spectrum of thep-fluorotoluene–argon van der Waals complex was analyzed with a molecular beam Fourier transform microwave spectrometer. In the frequency splitting of molecular transitions caused by the internal rotation of the methyl group with respect to the aromatic ring is contained the structure and barrier information. Elucidation is reduced by the analysis of internal rotors direction with respect to the principal axis system of the complex the number of possible solutions in the structure. Anrsandr0structure of the complex was calculated from these data. The argon is located 3.541(1) Å above the aromatic ring. By forming the complex, the barrier height of internal rotation is changed. An additionalV3term in the potential function occurs in the complex because the molecular symmetry of thep-fluorotoluene, containing aV6but noV3term, is reduced by the argon. For assumedV6= 144.79(19) GHz or 57.777(76) J/mol,V3is found to equal 552.0(10) GHz or 220.31(40) J/mol.