Structural and conformational properties of 1,1,1-trifluoro-2-propanol investigated by microwave spectroscopy and quantum chemical calculations

The microwave spectrum of 1,1,1-trifluoro-2-propanol, CF(3)CH(OH)CH(3), and one deuterated species, CF(3)CH(OD)CH(3), have been investigated in the 20.0-62.0 GHz spectral region at about -50 degrees C. The rotational spectrum of one of the three possible rotameric forms was assigned. This conformer is stabilized by an intramolecular hydrogen bond formed between the hydrogen atom of the hydroxyl group and the nearest fluorine atoms. The hydrogen bond is weak and assumed to be mainly a result of attraction between the O-H and the C-F bond dipoles, which are nearly antiparallel. The identified rotamer is at least 3 kJ/mol more stable than any other rotameric form. Two vibrationally excited states belonging to two different normal modes were assigned for this conformer, and their frequencies were determined by relative intensity measurements. The microwave work has been assisted by quantum chemical computations at the MP2/cc-pVTZ and B3LYP/6-311++G** levels of theory, as well as by the infrared spectrum of the O-H stretching vibration.     

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 relaxation of S-(+)-carvone and R-(+)-limonene studied by microwave Fourier transform spectroscopy and quantum chemical calculations

S-(+)-carvone (C₁₀H₁₄O, 5-isopropenyl-2-methylcyclohex-2-en-1-one) and R-(+)-limonene (C₁₀H₁₆, 4-isopropenyl-1-methylcyclohexene) have been characterized in the gas phase using a Fourier transform microwave spectrometer coupled to a supersonic molecular beam. Two conformers—with the isopropenyl group in the equatorial position—have been detected for each compound and described by a set of molecular parameters including the principal rotational constants and the quartic centrifugal distortion parameters. Quantum chemical calculations indicate that a third conformer might not be observed due to relaxation processes in the jet. The gas phase results are compared with the liquid phase IR-Raman-VCD spectra.     

Conformational properties and electronic structure of tetrahydrotetrazines studied by photoelectron spectroscopy and quantum chemical calculations

The photoelectron (PE) spectra of tetrahydro-1,2,3,4-tetrazines 1 and 2 and tetrahydro-1,2,4,5-tetrazines 3–5 have been recorded and their conformations have been investigated by ab initio SCF calculations. While v-tetrazine2 is planar, tetrazines 1 and 3–5 each possess two low-energy conformations, according to ab initio HF and Becke3LYP methods. Attempts to assign ionization potentials to molecular orbitals obtained by semiempirical PM3 calculations indicate that this method is not suited for the compounds studied. Best results were obtained when the ab initio hybrid method Becke3LYP of the density functional theory was employed. Two conformers of 1 and 3–5 are present in the gas phase and their PE spectra are superimposed one upon the other. For v-tetrazine1, ionizations arising from half-chair and unsymmetrical boat conformers have similar energies and cannot be separated in the PE spectrum. For s-tetrazine3, on the other hand, the spectrum clearly shows different ionizations of both half-chairs, 3ee and 3ae.     

Conformation and intramolecular hydrogen bonding of 2-chloroacetamide as studied by microwave spectroscopy and quantum chemical calculations

The microwave spectrum of 2-chloroacetamide (ClCH2CONH2) has been investigated at room temperature in the 19-80 spectral range. Spectra of the 35ClCH2CONH2 and 37ClCH2CONH2 isotopomers of one conformer, which has a symmetry plane (Cs symmetry), were assigned. The amide group is planar, and an intramolecular hydrogen bond is formed between the chlorine atom and the nearest hydrogen atom of the amide group. The ground vibrational state, six vibrationally excited states of the torsional vibration about the CC bond, as well as the first excited state of the lowest bending mode were assigned for the 35ClCH2CONH2 isotopomer, whereas the ground vibrational state of 37ClCH2CONH2 was assigned. The CC torsional fundamental vibration has a frequency of 62(10) cm(-1), and the bending vibration has a frequency of 204(30) cm(-1). The rotational constants of the ground and of the six excited states of the CC torsion were fitted to the potential function Vz = 16.1( + 2.3) cm(-1), where z is a dimensionless parameter. This function indicates that the equilibrium conformation has Cs symmetry. Rough values of the chlorine nuclear quadrupole coupling constants were derived as chi(aa) = -47.62(52) and chi(bb) = 8.22(66) MHz for the 35Cl nucleus and chi(aa) = -34.6(10) and chi(bb) = 6.2(11) MHz for the 37Cl nucleus. Ab initio and density functional theory quantum chemical calculations have been performed at several levels of theory to evaluate the equilibrium geometry of this compound. The density functional theory calculations at the B3LYP/6-311++G(3df,2pd) and B3LYP/cc-pVTZ levels of theory as well as ab initio calculations at the MP2(F)/cc-pVTZ level predict correct lowest-energy conformation for the molecule, whereas the ab initio calculations at the QCISD(FC)/6-311G(d) and MP2(F)/6-311++G(d,p) levels predict an incorrect equilibrium conformation.     

Structural and conformational properties of 1,2-propadienylphosphine (allenylphosphine) studied by microwave spectroscopy and quantum chemical calculations

1,2-Propadienylphosphine (allenylphosphine), H(2)C=C=CHPH(2), has been investigated by Stark and Fourier transform microwave spectroscopy. Two rotameric forms denoted syn and gauche have been assigned. The syn form has a symmetry plane (C(s)() symmetry) where the lone electron pair of phosphorus points toward the double bonds. The phosphino group is rotated roughly 120 degrees from this position in the gauche rotamer. The dipole moment of syn was determined to be mu(a) = 1.613(23), mu(b) = 2.347(24), mu(c) = 0 (for symmetry reasons), and mu(tot) = 2.848(28) x 10(-30) C m [0.854(8) D]. The energy difference between the two forms was found to be 2.1(4) kJ/mol from relative intensity measurements with syn as the more stable conformer. Extensive quantum chemical calculations have been carried out and accurate equilibrium structures have been determined for these two rotamers, as well as for the corresponding two conformers of vinylphosphine (H(2)C=CHPH(2)).     

Structural and conformational properties of 4-Pentyn-1-ol as studied by microwave spectroscopy and quantum chemical calculations

The microwave spectra of 4-pentyn-1-ol, HO(CH2)3C triple bond CH, and one deuterated species (DO(CH2)3C triple bond CH) have been investigated in a Stark spectrometer in the 17.5-80 GHz spectral region at about 0 degrees C, as well as in a pulsed-nozzle Fourier transform spectrometer in the 2.5-14 GHz range. A total of 14 spectroscopically different all-staggered rotameric forms are possible for this compound. It has previously been assumed that a conformer stabilized by intramolecular hydrogen bonding predominates in the gas phase, but the microwave spectrum of this rotamer was not assigned and it is concluded that this form is not present in high concentrations. However, the microwave spectrum indicates that several forms are present, two of which denoted ag+g+ and ag+a were assigned in this work. In these two forms, the H-O-C-C chains of atoms have an antiperiplanar conformation and the O-C-C-C links are synclinal ("gauche"). The C-C-C-C triple bond CH link is synclinal in ag+g+ but antiperiplanar in ag+a. The ag+g+ form is determined to be 1.5(6) kJ/mol more stable than ag+a by relative intensity measurements. The microwave study was augmented by quantum chemical calculations at the MP2/6-311++G** and G3 levels of theory. Both these quantum chemical procedures indicate that there are small energy differences between several rotametric forms, in agreement with the microwave findings. Both methods predict that ag+g+ is the global minimum.     

Conformational landscape in chiral terpenes from vibrational spectroscopy and quantum chemical calculations: S-(+)-carvone

S-(+)-carvone (5-isopropenyl-2-methylcyclohex-2-en-1-one) is the primary component in the oil of caraway. Different experimental and theoretical works reveal that there are two possible conformers in which the isopropenyl group can be in equatorial or axial position. For each one, three rotamers were found theoretically, with the equatorial rotamers around 95% of the whole statistical population. In the current work, from a complete assignment of the IR and Raman spectra and the results obtained from the study of the VCD spectrum of the title compound, the three most stable rotamers have been detected experimentally in the liquid phase for the first time. The present work reveals that IR, Raman and VCD are helpful complementary techniques to characterize flexible systems, as terpenes, which present several conformers.     

Conformational landscape in chiral terpenes from vibrational spectroscopy and quantum chemical calculations: S-(+)-carvone

S-(+)-carvone (5-isopropenyl-2-methylcyclohex-2-en-1-one) is the primary component in the oil of caraway. Different experimental and theoretical works reveal that there are two possible conformers in which the isopropenyl group can be in equatorial or axial position. For each one, three rotamers were found theoretically, with the equatorial rotamers around 95% of the whole statistical population. In the current work, from a complete assignment of the IR and Raman spectra and the results obtained from the study of the VCD spectrum of the title compound, the three most stable rotamers have been detected experimentally in the liquid phase for the first time. The present work reveals that IR, Raman and VCD are helpful complementary techniques to characterize flexible systems, as terpenes, which present several conformers.     

XeCu covalent bonding in XeCuF and XeCuCl, characterized by fourier transform microwave spectroscopy supported by quantum chemical calculations

XeCu covalent bonding has been found in the complexes XeCuF and XeCuCl. The molecules were characterized by Fourier transform microwave spectroscopy, supported by MP2 ab initio calculations. The complexes were prepared by laser ablation of Cu in the presence of Xe and SF(6) or Cl(2) and stabilized in supersonic jets of Ar. The rotational constants and centrifugal distortion constants show the XeCu bonds to be short and rigid. The (131)Xe, Cu, and Cl nuclear quadrupole coupling constants indicate major redistributions of the electron densities of Xe and CuF or CuCl on complex formation which cannot be accounted for by simple electrostatic effects. The MP2 calculations corroborate the XeCu bond lengths and predict XeCu dissociation energies approximately 50-60 kJ mol(-)(1). The latter cannot be accounted for in terms of induction energies. The MP2 calculations also predict valence molecular orbitals with significant shared electron density between Xe and Cu and negative local energy densities at the XeCu bond critical points. All evidence is consistent with XeCu covalent bonding.     

Homogeneous decomposition mechanisms of diethylzinc by Raman spectroscopy and quantum chemical calculations

The gas-phase decomposition pathways of diethylzinc (DEZn), a common precursor for deposition of Zn-VI compounds, were investigated in detail. The homogeneous thermal decomposition of DEZn in N2 carrier was followed in an impinging-jet, up-flow reactor by Raman scattering. Density Functional Theory calculations were performed to describe the bond dissociation behavior using the model chemistry B3LYP/6-311G(d) to estimate optimal geometries and Raman active vibrational frequencies of DEZn, as well as anticipated intermediates and products. Comparison of the measured DEZn decomposition profile to that predicted by a 2-D hydrodynamic simulation revealed that simple bond dissociation between zinc and carbon atoms is the dominant homogeneous thermal decomposition pathway. The calculations suggest several reactions involving intermediates and Raman scattering experiments confirming the formation of the dimer (ZnC2H5)2. In a different set of experiments, photolysis of DEZn gave evidence for decomposition by beta-hydride elimination. The results suggest that beta-hydride elimination is a minor pathway for the gas-phase homogeneous pyrolysis of diethylzinc. A reasonable transition state during beta-hydride elimination was identified, and the calculated energies and thermodynamic properties support the likelihood of these reaction steps.     

Structural and conformational properties of 2-propenylgermane (allylgermane) studied by microwave and infrared spectroscopy and quantum chemical calculations

The structural and conformational properties of allylgermane have been investigated using Stark and Fourier transform microwave spectroscopies, infrared spectroscopy, and high-level quantum chemical calculations. The parent species H2C=CHCH2GeH3 was investigated by microwave spectroscopy and infrared spectroscopy, while three deuterated species, namely, H2C=CDCH2GeH3, H2C=CHCHDGeH3, and H2C=CHCH2GeD3, were studied only by infrared spectroscopy. The microwave spectra of the ground vibrational state as well as of the first excited state of the torsion vibration around the sp2-sp3 carbon-carbon bond were assigned for the 70Ge, 72Ge, and 74Ge isotopomers of one conformer. This rotamer has an anticlinal arrangement for the C=C-C-Ge chain of atoms. The infrared spectrum of the gas in the 500-4000 cm(-1) range has been assigned. No evidence of additional rotameric forms other than anticlinal was seen in the microwave and infrared spectra. Several different high-level ab initio and density functional theory calculations have been performed. These calculations indicate that a less stable form, having a synperiplanar conformation of the C=C-C-Ge link of atoms, may coexist with the anticlinal form. The energy differences between the synperiplanar and anticlinal forms were calculated to be 5.6-9.2 kJ/mol depending on the computational procedure. The best approximation of the equilibrium structure of the anticlinal rotamer was found in the MP2/aug-cc-pVTZ calculations. The barrier to internal rotation of the germyl group was found to be 6.561(17) kJ/mol, from measurements of the splitting of microwave transitions caused by tunneling of the germyl group through its threefold barrier.     

Conformational equilibrium and potential energy surface of 1-fluorobutane by microwave spectroscopy and Ab initio calculations

The rotational spectra of four (GT, TT, TG, and GG) of the five possible conformers of 1-fluorobutane have been assigned by combining free jet and conventional microwave spectroscopy. The geometry optimization was performed at the MP2 (full) level of theory with the 6-31G (d) and 6-311G (d, p) basis sets and by using the B3LYP (3df, 3pd) density functional method. The relative stability of the five rotamers is calculated at the QCISD (T)/6-311G (d, p) level of theory. In spite of the fact that ab initio calculations indicated the unobserved GG' conformer to be more stable than at least one of the observed conformers it was not possible to detect its rotational spectrum. GT and TG are the most and the least stable species, respectively. The rotational spectra of several vibrational satellites of the four conformers have been studied by conventional microwave spectroscopy. The overall conformational equilibrium is governed by the two-dimensional potential energy surface of the skeletal torsions MeC-CC and FC-CC, which have been evaluated by a flexible model analysis, based on the experimental values of the relative conformational and vibrational energy spacings, and on the shifts of second moments of inertia upon conformational change and vibrational excitation. The relative energy of the fifth stable conformer (GG') was determined to be 333 cm(-1) from flexible model calculations, and to be 271 cm(-1) from the most accurate ab initio calculations.     

Conformational identification of cyclohexylphosphine by microwave spectroscopy

The microwave spectra of cyclohexylphosphine have been recorded in the 18.0–26.5 GHz region. A-type rotational transitions have been assigned. The ground state rotational constants were determined to be A = 4153.75 ± 0.23, B = 1362.31 ± 0.01 and C = 1104.14 ± 0.01 MHz for C₆H₁₁PH₂, and A = 4030.03 ± 0.25, B = 1312.72 ± 0.01 and C = 1072.33 ± 0.01 MHz for C₆H₁₁PD₂. From the experimental rotational constants, it is suggested that the assigned spectra have resulted from the chair conformation with the gauche phosphine group in an equatorial position (CESG). This form is believed to be the most populated conformational isomer in the gas phase.     

Conformational and thermodynamic analysis of the COXIB scaffold using quantum chemical calculations

Selective cyclo‐oxygenase‐2 inhibitors (COXIBs) are prominent members of the nonsteroidal anti‐inflammatory drugs. The neutral and protonated COXIB scaffold has been subjected to molecular computations in the gas phase and implicit solvent to measure the relative changes in the thermodynamic functions, enthalpy (H_rel), potential energy (U_rel), Gibbs free energy (G_rel) and entropy (S_rel) induced by selected substituents. Conformational analysis of the COXIB scaffold indentified four pairs of atropisomeric conformers (from I, I′ to IV, IV′) associated with a molecular structure containing a double rotor system. All conformers had similar stability. Para‐substitution with substituents that cover a wide range of Hammett sigma values did not alter the geometries of the neutral COXIB conformers; however, the protonated COXIB scaffold was showed an increase in structural and thermodynamic perturbations due to inductive effects. Flexibility and structural resilience of the COXIB scaffold under the conditions studied herein could be an important feature of the COXIBs, especially considering the previously proposed flexibility of the cyclo‐oxygenase binding site. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Conformational landscape of l-threonine in neutral,acid and basic solutions from vibrational circular dichroism spectroscopy and quantum chemical calculations

The biological relevance of amino acids is well known. They can be used as zwitterionic, cationic or anionic forms according to the pH of the medium where they are. Thus, our aim herein was to study the conformational preference of the polar amino acid l-threonine [C₄H₉NO₃, (2S,3R)-2-amino-3-hydroxybutyric acid] under different pH conditions. A conformational study in an aqueous solution of the dissociation equilibrium of the amino acid l-threonine was carried out for this purpose. We recorded, at room temperature, the Mid-IR, Far-IR, Raman and VCD spectra of l-threonine from the aqueous solutions at pH values 5.70 (zwitterionic species), 1.00 (protonated species) and 13.00 (deprotonated species). The number of conformers found with the conformational search was 9 zwitterions, 27 anions and 52 cations. Both the study of the conformational landscape and the theoretical analysis of the vibrational features were accomplished by using DFT and ab initio calculations, that is, B3LYP/6-311++G(d,p) level of theory for all the conformers obtained from the conformational search, M062X/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory for the most stable conformers. The presence of water was included with the IEF-PCM implicit hydration model. With regard to the zwitterion, the importance of the analysis of the low frequency region (700–30 cm^–1) in the Far-IR spectra should be noted, because it provides relevant information that can be used to determine the presence of the most stable structures.     

Al(III) complexation by alizarin studied by electronic spectroscopy and quantum chemical calculations

The complexation reaction of Al(III) by alizarin (Az), in methanol solution, has been followed by electronic absorption spectroscopy. Chemometric methods applied to the spectra set have shown the formation of two complexes of stoichiometry 1:1 and 2:1, with stability constants of 6.44 and 11.61, respectively. In the alizarin ligand, the fixation of Al(III) can occur either with the hydroxy-keto site or the catechol site. The comparison between the experimental spectrum of the 1:1 complex and those calculated with time dependent density functional theory, from different hypothetical complex structures, has shown that the first site involved in the Al(III) fixation is the catecholate function. Quantum chemical calculations have also allowed a complete assignment of Az and its 1:1 complex electronic spectra. For both, the observed transitions have essentially a π → π^∗ character. For the complexed form, only intra-ligand charge transfers are observed. The chelation of Al(III) engendered some conformational modifications of the ligand, notably at the complexation site level but also at the level of the intermediate ring of Az.     

Conformational properties of penicillins: quantum chemical calculations and molecular dynamics simulations of benzylpenicillin

Herein, we present theoretical results on the conformational properties of benzylpenicillin, which are characterized by means of quantum chemical calculations (MP2/6-31G* and B3LYP/6-31G*) and classical molecular dynamics simulations (5 ns) both in the gas phase and in aqueous solution. In the gas phase, the benzylpenicillin conformer in which the thiazolidine ring has the carboxylate group oriented axially is the most favored one. Both intramolecular CH. O and dispersion interactions contribute to stabilize the axial conformer with respect to the equatorial one. In aqueous solution, a molecular dynamics simulation predicts a relative population of the axial:equatorial conformers of 0.70:0.30 in consonance with NMR experimental data. Overall, the quantum chemical calculations as well as the simulations give insight into substituent effects, the conformational dynamics of benzylpenicillin, the frequency of ring-puckering motions, and the correlation of side chain and ring-puckering motions.     

Structural and conformational properties and intramolecular hydrogen bonding of (methylenecyclopropyl)methanol, as studied by microwave spectroscopy and quantum chemical calculations

The microwave spectra of (methylenecyclopropyl)methanol (H(2)C=C(3)H(3)CH(2)OH) and one deuterated species (H(2)C=C(3)H(3)CH(2)OD) have been investigated in the 20-80 GHz spectral range. Accurate spectral measurements have been performed in the 40-80 GHz spectral interval. The spectra of two rotameric forms, denoted conformer I and conformer IX, have been assigned. Both these rotamers are stabilized by intramolecular hydrogen bonds formed between the hydrogen atom of the hydroxyl group and the pseudo-pi electrons on the outside of the cyclopropyl ring, the so-called "banana bonds". The carbon-carbon bond lengths in the ring are rather different. The bonds adjacent to the methylene group (H(2)C=) are approximately 7 pm shorter that the carbon-carbon bond opposite to this group. It is found from relative intensity measurements of microwave transitions that conformer IX, in which the hydrogen bond is formed with the banana bonds of the long carbon-carbon bond, is 0.4(3) kJ/mol more stable than conformer I, where the hydrogen bond is formed with the pseudo-pi electrons belonging to the shortest carbon-carbon bond of the ring. The microwave study has been augmented by quantum chemical calculations at the MP2/6-311++G, G3 and B3LYP/6-311++G levels of theory.     

Conformational properties of 1-methyl-1-germacyclohexane: low-temperature NMR and quantum chemical calculations

The conformational preference of the methyl group of 1-methyl-1-germacyclohexane was studied experimentally in solution (low-temperature ¹³C NMR) and by quantum chemical calculations (CCSD(T), MP2 and DFT methods). The NMR experiment resulted in an axial/equatorial ratio of 44/56 mol% at 114 K corresponding to an A value (A = G _ax ? G _eq) of 0.06 kcal mol^?1. An average value for ΔG _e→a ^#  = 5.0 ± 0.1 kcal mol^?1 was obtained for the temperature range 106–134 K. The experimental results are very well reproduced by the calculations. CCSD(T)/CBS calculations + thermal corrections resulted in an A value of 0.02 kcal mol^?1, whereas a ΔE value of ?0.01 kcal mol^?1 at 0 K was obtained.