Structural studies on ethyl isovalerate by microwave spectroscopy and quantum chemical calculations

We observed the microwave spectrum of ethyl isovalerate by molecular beam Fourier transform microwave spectroscopy. The rotational and centrifugal distortion constants of the most abundant conformer were determined. Its structure was investigated by comparison of the experimental rotational constants with those obtained by ab initio methods. In a first step, the rotational constants of various conformers were calculated at the MP2/6-311++G** level of theory. Surprisingly, no agreement with the experimental results was found. Therefore, we concluded that in the case of ethyl isovalerate more advanced quantum chemical methods are required to obtain a reliable molecular geometry. Ab initio calculations carried out at MP3/6-311++G**, MP4/6-311++G**, and CCSD/6-311++G** levels and also density functional theory calculations using the B3LYP/6-311++G** method gave similar results for the rotational constants, but they were clearly distinct from those obtained at the MP2/6-311++G** level. With use of these more advanced methods, the rotational constants of the lowest energy conformer were in good agreement with those obtained from the microwave spectrum.     

Conformational Transformations of Sulfur‐Containing Rings: 2‐Methyltetrahydrothiophene Gas‐Phase Structures

Stable conformations of five‐member rings with the prototype cyclopentane are well‐known to exist as twist or envelope structures and are of general interest in chemistry. Here, we report on the conformational analysis of the sulfur‐containing ring 2‐methyltetrahydrothiophene studied by a combination of molecular beam Fourier transform microwave (MB‐FTMW) spectroscopy and quantum chemistry. Two twist conformers were observed, whereby highly accurate molecular parameters could be determined. In addition, the ³⁴S‐isotopologue of the most stable conformer was assigned in natural abundances. Geometry optimizations were performed at different levels of theory and the calculated rotational constants were compared with experimental values. Two transition states optimized at the MP2/6‐311++G(d,p) level using the Berny algorithm could illustrate the intramolecular conversion between both conformers.     

Microwave spectrum, conformational composition, and intramolecular hydrogen bonding of (2-chloroethyl)amine (ClCH2CH2NH2)

The microwave spectrum of (2-chloroethyl)amine, ClCH(2)CH(2)NH(2), has been investigated in the 22-120 GHz region. Five rotameric forms are possible for this compound. In two of these conformers, denoted I and II, the Cl-C-C-N chain of atoms is antiperiplanar, with different orientations of the amino group. The link of the said atoms is synclinal in the three remaining forms, III-V, which differ with respect to the orientation of the amino group. The microwave spectra of four of these conformers, I-IV, have been assigned. In two of these rotamers, III and IV, the amino group is oriented in such a manner that rare and weak five-membered N-H···Cl intramolecular hydrogen bonds are formed. The geometries of conformers I and II preclude a stabilization by this interaction. The energy differences between the conformers were obtained from relative intensity measurements of spectral lines. The hydrogen-bonded conformer IV represents the global energy minimum. This rotamer is 0.3(7) kJ/mol more stable than the other hydrogen-bonded conformer III, 4.1(11) kJ/mol more stable than II, and 5.5(15) kJ/mol more stable than I. The spectroscopic work has been augmented by quantum chemical calculations at the CCSD/cc-pVTZ and MP2/6-311++G(3df,3pd) levels of theory. The CCSD rotational constants and energy differences are in good agreement with their experimental counterparts.     

Spectroscopic and quantum chemical study of cyclopropylmethylphosphine, a candidate for intramolecular hydrogen bonding

The properties of the novel compound cyclopropylmethylphosphine (C3H5CH2PH2) have been investigated by means of Stark-modulation microwave spectroscopy and high-level quantum chemical calculations. Spectra attributable to the three conformers of the molecule with a synclinal arrangement of the H-C-C-P atoms were recorded and assigned. The experimental rotational constants obtained for these conformers were found to be in good agreement with those generated by ab initio geometry optimizations at the MP2/aug-cc-pVTZ level of theory. An estimate of the relative energies of the three conformers with observable spectra, by means of relative intensity measurements, compared favorably with the results of G3 energy calculations performed for the molecule. In addition to the observation of ground-state rotational spectra for three conformers, spectra belonging to a number of vibrationally excited states were also assigned with the aid of radio frequency microwave double-resonance experiments. A tentative assignment of these excited-state spectra was proposed by appealing to the results of density functional theory vibrational frequency calculations performed at the B3LYP/6-311++(3df,2pd) level. The energetically preferred conformer of the molecule allowed a close approach between a hydrogen atom belonging to the phosphino group and the edge of the cyclopropyl ring. The possibility of the formation of an intramolecular hydrogen bond to electron density associated with so-called banana bonds is discussed.     

Aggregation of 2‐Aminobenzimidazole—A Combined Experimental and Theoretical Investigation

An investigation of 2‐aminobenzimidazole was carried out by calculations at HF, MP2, and DFT levels of theory and also by UV and IR spectroscopy. The quantum chemical calculations predict a full shift of the equilibrium towards the amino form, but the absorption spectra in different solvents distinctly show a two‐component equilibrium system. Examination of possible equilibria in solution shows that an equilibrium between two dimeric forms of the amino tautomer of 2‐aminobenzimidazole explains the spectral observations.     

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.     

A microwave spectroscopic and quantum chemical study of 3-butyne-1-selenol (HSeCH2CH2C[triple bond]CH)

The microwave spectrum of 3-butyne-1-selenol has been studied by means of Stark-modulation microwave spectroscopy and quantum chemical calculations employing the B3LYP/aug-cc-pVTZ and MP2/6-311++G(3df,3pd) methods. Rotational transitions attributable to the H80SeCH2CH2C[triple bond]CH and H78SeCH2CH2C[triple bond]CH isotopologues of two conformers of this molecule were assigned. One of these conformers possesses an antiperiplanar arrangement for the atoms Se-C-C-C, while the other is synclinal and seems to be stabilized by the formation of a weak intramolecular hydrogen bond between the hydrogen atom of the selenol group and the pi electrons of the CC triple bond. The energy difference between these conformers was determined to be 0.2(5) kJ/mol by relative intensity measurements, and the hydrogen-bonded form was slightly lower in energy.     

Molecular Self‐Recognition: Rotational Spectra of the Dimeric 2‐Fluoroethanol Conformers

Fluoroalcohols show competitive formation of intra‐ and intermolecular hydrogen bonds, a property that may be crucial for the protein‐altering process in a fluoroalcohol/water solution. In this study, we examine the intra‐ and intermolecular interactions of 2‐fluoroethanol (FE) in its dimeric conformers by using rotational spectroscopy and ab initio calculations. Three pairs of homo‐ and heterochiral dimeric FE conformers are predicted to be local minima at the MP2/6‐311++G(d,p) level of theory. They are solely made of the slightly distorted most stable G+g?/G?g+ FE monomer units. Jet‐cooled rotational spectra of four out of the six predicted dimeric conformers were observed and unambiguously assigned for the first time. All four observed dimeric conformers have compact geometries in which the fluoromethyl group of the acceptor tilts towards the donor and ensures a large contact area. Experimentally, the insertion of the O? H group of one FE subunit into the intramolecular O? H???F bond of the other was found to lead to a higher stabilisation than the pure association through an intermolecular O? H???O? H link. The hetero‐ and homochiral combinations were observed to be preferred in the inserted and the associated dimeric conformers, respectively. The experimental rotational constants and the stability ordering are compared with the ab initio calculations at the MP2 level with the 6‐311++G(d,p) and aug‐cc‐pVTZ basis sets. The effects of fluorination and the competing inter‐ and intramolecular hydrogen bonds on the stability of the dimeric FE conformers are discussed.     

Synthesis of a new 1,2,3,4,5‐pentasubstituted cyclohexanol and determining its stereochemistry by NMR spectroscopy and quantum‐chemical calculations

The presence of substituents in cyclohexane can influence to the ratio of conformers; for some cases, the boat form is preferable. The new six‐membered cyclohexanol derivative 2 has been obtained by the synthesis of (E)‐1‐(bromophenyl)‐3‐phenylpropen‐2‐one ( 1 ). The NMR and quantum‐chemical conformational analysis for the 2 have carried out, and its possible mechanism of formation was given. Copyright © 2015 John Wiley & Sons, Ltd.     

An approach to estimate the energy and strength of the intramolecular hydrogen bond in different conformers of 4‐methylamino‐3‐penten‐2‐one

The molecular structure and intramolecular hydrogen bond energy of 32 conformers of 4‐methylamino‐3‐penten‐2‐one were investigated at MP2 and B3LYP levels of theory using the standard 6–31G** basis set and AIM analyses. Furthermore, calculations for all the possible conformations of 4‐methylamino‐3‐penten‐2‐one in water solution were also carried out at B3LYP/6–31G** level of theory. The calculated geometrical parameters and conformational analyses in gas phase and water solution show that the ketoamine conformers of this compound are more stable than the other conformers (i.e., enolimine and ketoimine). This stability is mainly due to the formation of a strong N? H···O intramolecular hydrogen bond, which is assisted by π‐electrons resonance. Hydrogen bond energies for all conformers of 4‐methylamino‐3‐penten‐2‐one were obtained from the related rotamers method. The nature of intramolecular hydrogen bond existing within 4‐methylamino‐3‐penten‐2‐one has been investigated by means of the Bader theory of atoms in molecules, which is based on topological properties of the electron density. The results of these calculations support the results which obtained by related rotamers method. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Structural Distortion of the Epoxy Groups in Norbornanes: A Rotational Study of exo‐2,3‐Epoxynorbornane

Exo‐2,3‐epoxynorbornane is studied in the gas phase by pulsed jet Fourier transform microwave spectroscopy in the 4–18 GHz region. Six isotopologues were observed and characterized in their natural abundance. The experimental substitution and effective structures were obtained. Comparison with the structure of norbornane shows significant differences in several bond lengths and valence angles upon introduction of the epoxy group. All the work is supported by quantum chemical calculations.     

Microwave spectrum and conformational composition of 2-fluoroethylisocyanide

The microwave spectrum of 2-fluoroethylisocyanide, FCH(2)CH(2)N≡C, has been investigated in the whole 50-120 GHz spectral region. Selected portions of the spectrum in the range of 18-50 GHz have also been recorded. The microwave spectra of the ground state and vibrationally excited states of two conformers have been assigned. Accurate spectroscopic constants have been derived from a large number of microwave transitions. The F-C-C-N chain of atoms is antiperiplanar in one of these rotamers and synclinal in the second conformer. The energy difference between the two forms was obtained from relative intensity measurements. It was found that the synclinal conformer is favored over the antiperiplanar form by 0.7(5) kJ/mol. Quantum chemical calculations at the high CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory were performed. Most, but not all, of the spectroscopic constants predicted in these calculations are in good agreement with the experimental counterparts. The theoretical calculations correctly indicate that the F-C-C-N dihedral angle in the synclinal form is about 67° but underestimate the magnitude of the gauche effect and erroneously predict the antiperiplanar rotamer to be 1.3-1.6 kJ/mol more stable than the synclinal conformer.     

Solvent and structural effects on the kinetics of the reactions of 2‐substituted cyclohex‐1‐enylcarboxylic and 2‐substituted benzoic acids with diazodiphenylmethane

The rate constants for the reaction of 2‐methyl‐cyclohex‐1‐enylcarboxylic, 2‐phenylcyclohex‐1‐enylcarboxylic, and 2‐methylbenzoic and 2‐phenylbenzoic acids with diazodiphenyl‐methane were determined in 14 various solvents at 30°C. To explain the kinetic results through solvent effects, the second‐order rate constants of the examined acids were correlated using the Kamlet–Taft solvatochromic equation. The correlations of the kinetic data were carried out by means of multiple linear regression analysis, and the solvent effects on the reaction rates were analyzed in terms of initial and transition state contributions. The quantitative relationship between the molecular structure and the chemical reactivity has been discussed, as well as the effect of geometry on the reactivity of the examined molecules. The geometric data of all the examined compounds corresponding to the energy minima in solvent, simulated as dielectric continuum, obtained using semiempirical MNDO‐PM3 energy calculations. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 664–671, 2007     

A microwave and quantum chemical study of the conformational properties and intramolecular hydrogen bonding of 1-fluorocyclopropanecarboxylic acid

The structural and conformational properties of 1-fluorocyclopropanecarboxylic acid have been explored by microwave spectroscopy and a series of ab initio (MP2/6-311++G(d,p) level), density functional theory (B3LYP/aug-cc-pVTZ level), and G3 quantum chemical calculations. Four "stable" conformers, denoted conformers I-IV, were found in the quantum chemical calculations, three of which (conformers I -III) were predicted to be low-energy forms. Conformer I was in all the quantum chemical calculations predicted to have the lowest energy, conformer III to have the second lowest energy, and conformer II to have the third lowest energy. Conformers II and III were calculated to have relatively large dipole moments, while conformer I was predicted to have a small dipole moment. The microwave spectrum was investigated in the 18-62 GHz spectral range. The microwave spectra of conformers II and III were assigned. Conformer I was not assigned presumably because its dipole moment is comparatively small. Conformer II is stabilized by an intramolecular hydrogen bond formed between the fluorine atom and the hydrogen atom of the carboxylic acid group. Conformer III has a synperiplanar orientation for the F-C-C=O and H-O-C=O chains of atoms. Its dipole moment is: mua = 3.4(10), mub = 10.1(13), and muc = 0.0 (assumed) and mu(tot) = 10.6(14) x 10(-30) C m [3.2(4) D]. Several vibrationally excited states of the lowest torsional mode of each of II and III were also assigned. The hydrogen-bonded conformer II was found to be 2.7(2) kJ/mol less stable than III by relative intensity measurements. Absolute intensity measurements were used to show that the unassigned conformer I is the most abundant form present at a concentration of roughly 65% at room temperature. Conformer I was estimated to be ca. 5.0 kJ/mol more stable than the hydrogen-bonded rotamer (conformer II) and ca. 2.3 kJ/mol more stable than conformer III. The best agreement with the theoretical calculations is found in the MP2 calculations, which predict conformer I to be 5.1 kJ/mol more stable than III and 1.7 kJ/mol more stable than II.     

Intramolecular hydrogen bond in 3‐imino‐propenylamine isomers: AIM and NBO studies

The molecular structure and intramolecular hydrogen bond energy of 18 conformers of 3‐imino‐propenyl‐amine were investigated at MP2 and B3LYP levels of theory using the standard 6‐311++G** basis set. The atom in molecules or AIM theory of Bader, which is based on the topological properties of the electron density (ρ), was used additionally and the natural bond orbital (NBO) analysis was also carried out. Furthermore calculations for all possible conformations of 3‐imino‐propenyl‐amin in water solution were also carried out at B3LYP/6‐311++G** and MP2/6‐311++G** levels of theory. The calculated geometrical parameters and conformational analyses in gas phase and water solution show that the imine–amine conformers of this compound are more stable than the other conformers. B3LYP method predicts the IMA‐1 as global minimum. This stability is mainly due to the formation of a strong N? H···N intramolecular hydrogen bond, which is assisted by π‐electrons resonance, and this π‐electrons are established by NH2 functional group. Hydrogen bond energies for all conformers of 3‐imino‐propenyl‐amine were obtained from the related rotamers methods. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

DFT‐Aided Vibrational Circular Dichroism Spectroscopy Study of (−)‐S‐cotinine

The implementation of a strategy comprising the use of vibrational circular dichroism spectroscopy and DFT calculations allows determination of the solution‐state conformational distribution in (?)‐S‐cotinine, giving further proof of the extra conformer‐discriminating potential of this experimental technique, which may offer unique molecular fingerprints of subtly dissimilar molecular conformers of chiral samples. Natural bond orbital electronic structure calculations of the rotational barrier height between the two main conformers of the species indicate that hyperconjugative effects are the key force governing the conformational equilibrium. The negligible effect of the solvent’s polarity over both structure and conformational energy profile supports this result.     

Conformational composition of cyclopentadienylphosphine investigated by microwave spectroscopy and quantum chemical calculations

The properties of cyclopentadienylphosphine have been investigated by means of Stark-modulation microwave spectroscopy and quantum chemical calculations at the MP2/aug-cc-pVTZ, B3LYP/6-311++G(d,p), and G3 levels of theory. Spectra attributable to two rotamers denoted conformers I and II have been assigned. Conformer I has a symmetry plane (Cs symmetry) consisting of the bisectors of the cyclopentadiene ring and of the phosphino group with the lone electron pair of phosphorus pointing toward the carbon ring. In conformer II, the phosphino group is rotated approximately 120 degrees out of this plane. Relative intensity measurements have been made, and it was found that conformer II is more stable than I by 1.3(4) kJ/mol. The preferred conformer represents a borderline case of intramolecular hydrogen bond stabilization. The experimental and MP2/ aug-cc-pVTZ rotational constants differ by several percent, which indicates that the aug-cc-pVTZ basis set is not large enough to be able to predict an accurate structure for the two conformers that are close to the equilibrium geometries. 5-Substituted 1,3-cyclopentadienyl derivatives may undergo circumambulatory rearrangements. However, there is no manifestation of this effect in the microwave spectrum of cyclopentadienylphosphine.     

Quantum scattering calculation for reaction Br + H2 on two potential energy surfaces

Three‐dimensional time‐dependent quantum wave packet calculations have been carried out for Br + H₂ on a new global ab initio and a semi‐empirical extended London–Eyring–Polanyi–Sato potential energy surface. It is shown that on the ab initio surface, the threshold energy is much lower, and the reaction probabilities, cross sections, and rate constants are much larger. The effects of the initial rovibrational excitation have also been studied. Comparison of rate constants with experimental measurement implies that the ab initio surface is more suitable for quantum dynamic calculation. The possible reasons and mechanism for the dynamical difference on the two PES are analyzed and discussed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007