Conformational stability and vibrations of aminopropylsilanol molecule

Density functional theory (DFT), using the B3-LYP/6-31G(d,p) method have been used to investigate the conformation and vibrational spectra of aminopropylsilanetriol (APST) NH2CH2CH2CH2Si(OH)3. The potential function for CCCSi torsion gives rise to two distinct conformers trans and gauche. The predicted energy of the more stable trans conformer is 337 cm-1 less than the energy of gauche conformer. The calculated barriers to the conformation interchange are: 1095, 2845 and 438 cm-1 for the trans to gauche, gauche to gauche and gauche to trans conformers, respectively. For the trans conformer the potential energy curve for the Si(OH)3 groups torsion in APST has been calculated changing the HOSiC dihedral angle. The barrier for the internal rotation of 3065 cm-1 has been obtained. The optimized molecular structure of APST dimer calculated for trans conformer has a SiOSi angle of 143.2 degrees, and a SiOSi bond length of 0.164 nm. A complete vibrational assignment for both conformers as well as for trans-dimer is supported by the normal coordinate analysis, calculated IR intensities as well as Raman activities. On the basis of the results, the vibrational spectra of APST aqueous solution and APST polymer have been analyzed. The average error between the observed and calculated frequencies is 14 cm-1.     

Conformer assignment of OH stretches in ethanol and isopropanol

Rigid harmonic asymetric top contour simulation is applied to assign the OH stretch transitions observed in the vapour phase infrared spectra of ethanol and isopropanol to gauche and trans conformers. For hetanol, the high frequency ν(OH) absorption must be assigned to the trans, for isopropanol the high frequency absorption is found to be due to the gauche conformer.     

Aziridinyl ketones and their heteroanalogs. 5. Synthesis and structures of 2-aryl-3-aroylaziridines

A number of trans- and cis-isomeric 1-R-2-aryl-3-aroylaziridines were synthesized, and their IR spectra were studied. Intramolecular hydrogen bonding is realized in the trans isomers when R = H, and they exist in the only possible conformation (intermediate between a gauche and a cisoid conformation). cis-Isomers II (R = alkyl) exist in solutions in the form of two conformers, viz., gauche and cisoid conformers, and the gauche conformer is thermodynamically preferable.See [1] for Communication A.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No, 11, pp. 1489–1494, November, 1980.     

Gauche- and trans-n-butane and their IR-induced interconversion in solid neon

Both conformers of n-butane are trapped from the gas phase in solid Ne at 4 K. Broad band infrared irradiation (mainly the CH-stretching region) induces interconversions, the quantum yield for the gauche — trans isomerization being approximately eight times higher than that for the reverse process. The energy difference between the conformers was found to be ?3.05 kJ mol^?1. The vibrational spectra of both conformers are given.     

Effect of ionization on infrared and electronic absorption spectra of methyl and ethyl formate in the gas phase and in astrophysical H2O ice: a computational study

This work reports infrared and electronic absorption spectra of trans and gauche conformers of neutral ethyl formate, trans and cis conformers of neutral methyl formate, their ions in the gas phase, and neutral ethyl and methyl formate in astrophysical H(2)O ice. The second-order M?ller-Plesset perturbation (MP2) method with TZVP basis set has been used to obtain ground-state geometries. An influence of ice on vibrational frequencies of neutral ethyl and methyl formate was obtained using integral equation formalism polarizable continnum model (IEFPCM). Significant shift in vibrational frequencies for neutral methyl and ethyl formate when studied in H(2)O ice and upon ionization is observed. Rotational and distortion constants for neutral ethyl and methyl formate from this work are in excellent agreement with the available experimental values. Electronic absorption spectra of conformers of ethyl and methyl formate and their ions are obtained using time-dependent density functional method (TDDFT). The nature of electronic transitions is also identified. We suggested lines especially good to detect these molecules in interstellar medium. Using these lines, we can identify the conformers of ethyl and methyl formate in gas phase and H(2)O ice in interstellar medium. This comparative study should provide useful guidelines to detect conformers of ethyl and methyl formate and their ions in gas phase and neutral molecules in H(2)O ice in different astronomical environment.     

Chlorocarbonyl trifluoromethanesulfonate, ClC(O)OSO2CF3: structure and conformational properties in the gaseous and condensed phases

Pure chlorocarbonyl trifluoromethanesulfonate, ClC(O)OSO(2)CF(3), has been prepared in about 58% yield by the ambient-temperature reaction between ClC(O)SCl and AgCF(3)SO(3). The conformational properties of the gaseous molecule have been studied by vibrational spectroscopy [IR(gas), IR(matrix), and Raman(liquid)] and quantum chemical calculations (HF and B3LYP with 6-31+G* basis sets); in addition, the solid-state structure has been determined by X-ray crystallography. ClC(O)OSO(2)CF(3) exists in the gas phase as a mixture of trans [ClC(O) group trans with respect to the CF(3) group] and gauche conformers, with the trans form being the more abundant [66(8)% from IR(matrix) measurements]. In both conformers, the C=O bond of the ClC(O) group is oriented synperiplanar with respect to the S-O single bond. The experimental free energy difference between the two forms, DeltaG degrees = 0.8(2) kcal mol(-1) (IR), is slightly smaller than the calculated value (1.0-1.5 kcal mol(-1)). The crystalline solid at 150 K [monoclinic, P2(1)/n, a = 7.3951(9) angstroms, b = 24.897(3) angstroms, c = 7.4812(9) angstroms, beta = 99.448(2) degrees, Z = 8] consists surprisingly of both trans and gauche forms. Whereas the more stable conformer for the more or less discrete molecules and the polarization effects would tend to favor the trans form, the packing effects would stabilize the gauche rotamer in the solid state.     

Chirped pulse Fourier transform microwave study of 2,2,2-trifluoroethyl formate

The rotational spectra of 2,2,2-trifluoroethyl formate and its three (13)C isotopologues have been measured with a molecular-beam-based, chirped-pulsed Fourier transform microwave spectrometer in combination with a conventional Balle-Flygare-type instrument up to 18 GHz. Although ab initio calculations predict the presence of two low-energy conformers (analogous to the trans and gauche forms of ethyl formate), the trans isomer was the only stable conformer observed. The r(s) geometry of the molecular main carbon frame was precisely derived based on a coplanar heavy-atom backbone of this conformer. ESPs of the two lowest energy conformers were calculated to obtain information about the role of through-space effects on their structures and relative stability.     

Conformational preferences of chiral molecules: free jet rotational spectrum of 1-phenyl-1-propanol

The rotational spectra of normal and O-d species of the two most stable conformers of chiral 1-phenyl-1-propanol, obtained by free jet millimetre-wave absorption spectroscopy reveal that both conformers are stabilized by a O-H[dot dot dot]pi interaction, and have the Calpha-Cbeta-bond oriented nearly perpendicular to the plane of the benzene ring. The methyl group is trans with respect to the phenyl group for the most stable conformer (T), while it is gauche with respect to the phenyl group and entgegen with respect to the hydroxyl group for the second most stable conformer (GE). The energy difference (E(GE)-E(T)) was estimated to be 50(50) cm(-1) from relative intensity measurements.     

The vibrational spectra of [15N2]-succinonitrile

For the first time, the infrared and Raman spectra of [15N2]-succinonitrile are presented and discussed in detail. Assignments of the vibrational bands of its two rotational conformers gauche and trans, respectively, have been made for both infrared and Raman spectra. The assignments were based on a recent ab-initio force field calculation for succinonitrile, taking into account the vibrational frequencies of other succinonitrile isotopomers. There are differences in the frequencies of the vibrational bands due to the mass increase in the cyanide groups, which have been analysed in depth.     

Spectra and structure of silicon-containing compounds. XXX. Raman and infrared spectra,conformational stability,vibrational assignment of chloromethyl silyl dichloride

The Raman spectra (3200-30 cm(-1)) of liquid and solid, and infrared spectra of gaseous and solid chloromethyl silyl dichloride, ClCH2SiHCl2, have been recorded. Variable temperature (-105 to -150 degrees C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference was determined to be 363 +/- 40 cm(-1) (4.34 +/- 0.48 kJ mol(-1)), with the more stable form being the gauche conformer, which is consistent with the prediction from ab initio calculations at both the Hartree-Fock level and with full electron correlation by the perturbation method to second order. It is estimated that 92% of the sample is in the gauche form at ambient temperature. A complete vibrational assignment is proposed for the gauche conformer and several of fundamentals of the trans conformer based on infrared band contours, relative intensities, depolarization values, and group frequencies, which is supported by normal coordinate calculations utilizing the force constants from the ab initio MP2/6-31G(d) calculations. The r0 SiH bond distances of 1.476 and 1.472 A have been obtained for the trans and gauche conformers, respectively, from the silicon-hydrogen stretching frequencies. The optimized geometries have also been obtained from ab initio calculations utilizing several different basis sets with full electron correlation by the perturbation method up to MP2/6-311 + G(2d,2p). The results are discussed and compared to some corresponding results for several related molecules.     

Rotational isomerism in 1,3-dimercaptopropane

It is well known that the molecules of 1,2-dihaloethanes [1], 1-halopropanes [2] and 1,2-dihalopropanes [3] exist in more than one rotational isomeric form trans (T) and gauche (G) in the liquid state and only in the trans (T) form in the crystalline state at low temperature. In the case of 1,3-dihalopropanes [4], however, the molecules exist in dynamic equilibrium of three types of rotameric forms, TT, TG and GG in the liquid phase, while GG is the only stable form in the crystalline phase. The number of rotational conformers present in 1,2-ethanedithiol [5], 1-propanethiol [6] and 1,2-propanedithiol [7] and their stabilities in the liquid and crystalline solid phases are observed to be similar to those of the corresponding haloalkanes [1–3]. Since such studies with 1,3-dimercaptopropane have not yet been made, its infrared spectrum in the liquid state and its Raman spectra in the liquid and solid states have been investigated. The present communication reports the results and their analyses with reference to the possibility of rotational isomerism in the molecule.     

The structure of the singlet tetramethylene diradical intermediate

Ab initio MC SCF geometry optimizations of the gauche and trans conformers of the singlet tetramethylene diradical have been carried out using MC SCF gradients with a minimal (STO-3G) and extended (4-31G) basis set. At both computational levels, it has been found that the tetramethylene diradical exists as a stable species in two different conformations, a gauche and a trans.     

Conformational study of tyramine and its water clusters by laser spectroscopy

Tyramine and its monohydrated clusters have been investigated by several laser spectroscopic methods in a pulsed molecular beam. The conformational structures and their effects on hydration have been revealed by resonant two-photon ionization (R2PI), UV-UV ion-dip, and ab initio calculations. UV rotational band contour spectra of the S1 <-- S0 origin bands enabled determination of ethylamine side chain conformations for all seven stable conformers of tyramine. When coexpanding tyramine with a mixture of Ar and water vapor, we have found two kinds of conformational effects on hydration. One is sensitive to conformation of the ethylamine chain and the other to the orientation of the OH group, particularly in the most stable pair of conformers. UV-UV ion-dip spectra detected seven stable conformers of the monohydrated clusters, of which hydrogen-bonding structures, spectral shifts, and origin band intensity distributions are well explained by considering tyramine as a hybrid of phenylethylamine (PEA) and phenol. Monohydration of the most stable gauche conformer pair (cis and trans) of tyramine leads to more detailed conformational assignments regarding the orientation of the phenolic OH group. Cyclic hydrogen-bonding linkage formed in the monohydrated cluster pair is found to be sensitive to the orientation of the phenolic OH group. One of the cluster pair, in which tyramine has the gauche-cis conformation, is more stabilized by the cyclic hydrogen bonding and its origin band intensity becomes stronger than that of the other.     

Conformational studies of 1,2,3-trichloro- and 1,2,3-tribromopropane by nuclear magnetic resonance spectroscopy: attractive parallel (1:3) halogen-hydr

Analysis of the NMR spectra of 1,2,3-trichloropropane and 1,2,3-tribromopropane in various media shows the most stable conformer to be AG_-. The populations of several conformera have been estimated by using pure trans and gauche coupling constants obtained from closely similar molecules. The calculated populations found in non-polar solvents agree well with those obtained by electron diffraction studies in the gas phase. It is suggested that the AG_- form is stabilised relative to AG₊ by the former having two parallel (1:3) halogen-hydrogen attractions against one in the AG₊ form. Comparison is made to related molecules where the most stable conformers also have the greatest number of parallel (1:3) halogen-hydrogen interactions.     

Internal rotation in 1,2-di-(p-XC6H4)ethanes (X=H, Br, NO2): infrared spectra and compensation effect

Infrared absorption spectra and internal rotation of 1,2-di-(p-XC(6)H(4))ethanes (X=H, Br, NO(2)) in crystalline phase, liquid and solutions at various temperatures have been investigated. Band fitting was applied to conformationally sensitive regions of the spectra, and assignment of the peaks to trans and gauche conformations was performed. Enthalpy and entropy differences of the conformers (deltaH(0) and deltaS(0)) were found to be solvent-dependent, and it is interpreted in terms of previously discovered compensation effect. The values deltaH(0) and deltaS(0) for 1,2-di-(p-NO(2)C(6)H(4))ethane obtained are unusually large.     

Is 2-cyclopropylpropene really gauche?

Infrared spectra of gaseous and solid 2-cyclopropylpropene (2-CPP, c-C3H5C (CH3)CH2) have been recorded from 3500 to 40 cm-1, and Raman spectra (3200-150 cm-1) of the liquid as well as mid-infrared spectra of 2-CPP in liquid krypton solution (from -105 to -150 degrees C) were also obtained. Ab initio calculations, with basis sets up to 6-311+G(2df, 2pd), were carried out for this molecule, using the restricted Hartree-Fock (RHF) approach, with full electron correlation by the perturbation method to second order (MP2(full)) and density functional theory (DFT) by the B3LYP method. The combination of the experimental and computational results (particularly with the higher basis sets) unequivocally identifies the more stable conformer of 2-CPP as the trans form, with the gauche rotamer higher in energy, but also stable. The cis structure of this compound is not observed experimentally, and is predicted by the computational approaches to be a transition state. By studying the temperature variation of two well-resolved sets of conformational doublets of 2-CPP dissolved in liquid krypton, an average enthalpy difference between conformers of 182+/-18 cm-1 (2.18+/-0.22 kJ mol-1) has been determined, with the trans conformation lower in energy in the fluid states, and the sole conformer present in the polycrystalline solid phase. This enthalpy difference corresponds to an ambient temperature conformational equilibrium in the fluid phases of 2-cyclopropylpropene containing approximately 55+/-2% of the more stable trans rotameric form. A complete vibrational assignment for the trans conformer of 2-CPP is given, and many of the bands of the gauche rotamer have also been assigned. Structural parameters, dipole moments, and rotational constants for this molecule have been calculated at the MP2(full)/6-311+G(d,p) level, and these results--as well as the results from the experimental studies--are compared to similar quantities in related compounds.     

Octa-1,3,5,7-tetraene: Ab initio geometries and vibrational spectra of some stable structures

Complete geometry optimizations of trans, Trans, trans, Trans, trans-, gauche, Trans, trans, Trans, trans-, trans, Trans, trans, Cis, trans-, and gauche, Trans, trans, Cis, trans-octa-1,3,5,7-te-traenes were carried out at the RHF/6-31G level. Characteristic changes in the geometry are found in going from the planar conformers of octatetraene to the corresponding higher-energy stable forms. The harmonic force constants were computed for the above conformers at the RHF/6-31G//RHF/6-31G level using analytical second derivatives. The computed force fields of these four molecules were then corrected using empirical scale factors transferred fromtrans-buta-1,3-diene. To account for the vibronic coupling effect, which may be a characteristic of oligoenes, a special scale factor was introduced for the two internal coordinates which correspond to stretching the central C=C double bonds. A complete assignment of the experimental spectra oft, T, t, T, t-octatetraene is also given.On leave from Laboratory of Molecular Spectroscopy, Department of Chemistry, Moscow State University, Moscow 119899, U.S.S.R.Preliminary results were reported at the Thirteenth Austin Symposium on Molecular Structure, Austin, TX, USA March 12–14, 1990, S 5, p. 91. and at the Second World Congress of the Theoretical Organic Chemists, Toronto, Canada July 8–14, 1990, BP-35 (Canada).     

Molecular conformers in gas-phase ethanol: A temperature study of vibrational overtones

Overtone absorption spectra are reported for ethanol vapor (10150–19900 cm^?1) measured by intracavity photoacoustic spectroscopy. The OH overtones are composed of two sub-bands which are assigned as the transitions of two conformers of the OH bond in the trans or gauche position with respect to the methyl group. From the temperature dependence of the OH overtone intensity we determine the enthalpy difference between the conformers to be 0.7 ± 0.1 kcal/mole.     

NMR Studies of Solvent Effect on Internal Rotation in Some Non-Symmetrical 1,2-Disubstituted Ethanes

The A₂B₂ system of PMR spectra of 3-bromopropionic acid, 3-chloropropionic acid, 3-bromopropionitrile, and 3-chloropropionitrile exhibit appreciable solvent effect at room temperature. NMR spectroscopic parameters of A₂B₂ spectrum as well as physical parameters related to internal rotation, i. e. the highest energy barrier and the energy difference between rotamers, were determined for these compounds in the medium of various solvents. It was found that in the case of 3-bromopropionic acid, the trans rotamer is more stable than the gauche rotamers, and the energy difference decreases with increasing dielectric constant of solvent. While in the case of 3-bromopropionitrile, the gauche rotamers were found to be more stable than the trans rotamer and the energy difference increases with increasing dielectric constant of solvent. In the remaining two compounds, 3-chloropropionic acid and 3-chloropropionitrile, both trans and gauche rotamers are equally stable in a solvent of low dielectric constant, however in a solvent of higher dielectric constant, the gauche rotamers become more stable than the trans rotamer and the energy difference becomes more pronounced with increasing dielectric constant of solvent.     

Solvent-assisted conformational isomerization and the conformationally-pure REMPI spectrum of 3-aminophenol

3-Aminophenol (3AP) has two conformers, cis and trans, depending on the orientation of the OH group relative to the NH(2) group. While both conformers are found in the jet-cooled spectra of 3AP, only the trans isomer was found in the REMPI spectrum of the 3AP(NH(3))(1) cluster. It was suggested that the cis conformer of the cluster isomerizes to the more stable trans conformer in the ground state during supersonic expansion. Solvent-assisted conformational isomerization (SACI) is believed to drive the population into the more stable trans isomer. SACI also occurs for the 3AP monomer, reducing 50% of the cis/trans ratio when the ammonia concentration in the expansion is higher than 0.1%. Depending on the expansion condition, the cis conformer can be completely depleted. When other solvents were introduced in the expansion, SACI occurred with only certain solvents whose binding energy is higher than the isomerization barrier. SACI can be used as a means to prepare the most stable conformer of gas phase biomolecules.