Potential Function For Internal Rotation Research Articles

Rotational isomerism in 1,2-difluoro-1,1,2-trichloro-ethane as studied by vibrational spectroscopy and ab initio calculations

Infrared and Raman spectra of 1,2-difluoro-1,1,2-trichloroethane (DFTCE) were measured in the gaseous, liquid and glassy states within a wide temperature interval. The complete assignment of the spectra was made on the basis of experimental information and theoretical (ab initio and DFT) force field calculations. It was found that in all aggregate states the substance existed as a mixture of three conformers, the most stable having trans-position of C–Cl bonds. Enthalpy differences between conformers in the gaseous phase were determined to be 1.93±0.25(t−g+),0.75±0.20(t−g−) and 1.09±0.20(g−−g+)kJmol−1. The potential function of internal rotation (PFIR) was estimated from the torsion fundamentals and calculated at the HF/6-31G∗ and B3LYP/6-31G∗ levels of theory.

Quantum Chemical Calculation of Conformations and Rotation Barriers of Functional Groups in Arylsulfonyl Halides

The semiempirical PM3 method is used to calculate the potential functions of internal rotation of the functional groups –SO2Cl, –NO2, –CH3, –OCH3, and –NH2 of benzenesulfonyl halide molecules (PhSO2Hal, Hal = F, Cl, Br, I) and twelve substituted derivatives of benzenesulfonyl chloride. Molecular conformations have been determined and internal rotation barriers of the functional groups have been calculated. For meta- and para-substituted benzenesulfonyl chlorides, the projection of the S–Hal bond is perpendicular to the plane of the benzene ring. The rotation barriers of the –SO2Hal group of benzenesulfonyl halides increase in the series Hal = F, Cl, Br, I. The rotation barriers of the –SO2Cl group of benzenesulfonyl chloride with meta- and para-substituents slightly increase with the electron-donor properties of the substituent. The rotation barriers of the functional groups of ortho-substituted benzenesulfonyl chlorides are 3 or 4 times as high as those of the meta- and para-isomers. For para-substituted benzenesulfonyl chlorides, the rotation barriers of the functional groups increase in the order –CH3, –NO2, –SO2Cl, –OCH3, –NH2.

Gas Phase Structure of Methyl Trifluoromethanesulfonate, CH3OSO2CF3, and Conformational Properties of Covalent Sulfonates

The molecular structure of methyl trifluoromethanesulfonate (CH3OSO2CF3, methyl triflate) was investigated by gas electron diffraction (GED) and quantumchemical calculations (HF/3-21G* and B3LYP/6-31G*). The GED analysis revealed a gauche conformation (methyl group gauche with respect to CF3 group) with a dihedral angle φ(C−O−S−C) = 89(7)°. This value is reproduced correctly by the calculations. Some calculated bond lengths and bond angles, however, differ from experimental values by up to ±0.07 Å and ±6°. Additional B3LYP/6-31G* calculations have been performed for some covalent sulfonates. The potential functions for internal rotation around the S−O single bond in FOSO2F and ClOSO2F possess minima for gauche and trans orientation of the O−X (X = F or Cl) bond. The trans minima are 1.6 and 1.3 kcal mol-1 higher in energy. The potential functions for triflic acid, HOSO2CF3, for CH3OSO2Cl and CH3OSO2F possess minima only for gauche structures and maxima for trans orientation. These results are in partial contrast to previous experimental studies.

S 1 ←S 0 vibronic spectrum and the structure of the chloral molecule in theS 1 stateS 0 vibronic spectrum and the structure of the chloral molecule in theS 1 state

The vibronic absorption spectrum of chloral (CCl3COH) vapors is studied in the region of S1 ← S0 electron transition (32,000–28,700 cm−1). The 29,070 cm−1 vibronic transition (not observed because of low intensity) is believed to be the ‘start’ of the electron transition. Several fundamentals are found in the S0 and S1 states. Inversion splitting of the zero vibrational level in the S1 state of chloral, indicating a nonplanar structure of the carbonyl fragment, is found. The intensity ratio of the torsional transition bands indicates that the S1 ← S0 electronic excitation of the chloral molecule causes significant changes in the orientation of the −CCl3 group relative to the molecular framework. The potential functions of internal rotation (S0 and S1 states) and inversion (S1 state) of the chloral molecule are determined from experimental data. The potential barriers of internal rotation (S0 and S1 states) and inversion (S1 state) are 380, 780, and 760 cm−1 (4.5, 9.3, and 9.1 kJ/mole), respectively.

Ab initio study of molecular structure and internal rotation in methyldicyanophosphine and methyldiisocyanophosphine. The Raman spectrum of methyldicyanophosphine and its interpretation with the use of scaling ofab initio force fields

The equilibrium geometric parameters and structures of transition states of internal rotation for the molecules of methyldicyanophospine MeP(CN)2 and its isocyano analog MeP(NC)2 were calculated by the RHF and MP2 methods with the 6–31G* and 6–31G** basis sets. At the MP2 level, the total energy of cyanide is −35 kcal mol−1 lower than that of isocyanide and the barriers to internal rotation of methyl group for MeP(CN)2 and MeP(NC)2 are 2.2 and 2.7 kcal mol−1, respectively. For both molecules, the one-dimensionalab initio potential functions of internal rotation approximated by a truncated Fourier series were used to determine the frequencies of torsional transitions by solving direct vibrational problems for a non-rigid model. The Raman spectrum of crystalline MeP(CN)2 was recorded in the range 3500–50 cm−1. The vibrational spectra of this compound were interpreted by scalingab initio force fields calculated by the RHF and MP2 method. The vibrational spectrum of methyldiisocyanophosphine was predicted with the use of the obtained scale factors.

Energy optimization, vibrational frequencies and asymmetric potential function for internal rotation in 2-methyl-2-butenoyl fluoride and chloride

The conformational stability and structure of 2-methyl-2-butenoyl fluoride and chloride were investigated by utilizing ab initio calculations with 6-31G ∗ basis set at RHF and MP2 levels. For both compounds, the s-trans conformer was predicted by the two levels to be slightly the lower energy form. Full optimization was performed at the transition states and the barriers to interconversion between the cis and trans conformers were calculated. The vibrational frequencies were computed at HF level and the zero-energy corrections were included into the calculated barriers. The inclusion of electron correlation in the calculations was shown to make a small change in the calculated energies and barriers, particularly in the case of the chloride.

S1←S0 vibronic spectrum and structure of fluoral in the S1 state

The vibronic absorption spectrum of fluoral vapor was studied in the region of the S1←S0 electronic transition (313–360 nm). The origin O00+) of the transition (29419 cm−1) and a number of fundamental frequencies in the S0 and S1 states were determined. The character of intensity distribution in the spectral bands indicates that the electronic excitation leads to significant change of the CF3 group orientation relative to the molecular frame. Moreover, it was found that the carbonyl fragment of the molecule in the S1 state has pyramidal structure (in contrast, the carbonyl fragment of the fluoral molecule in the S0 state is planar). The experimental torsion and inversion energy levels were used for the calculation of internal rotation and inversion potential functions of fluoral molecule in the S1 state. The potential barriers to internal rotation and inversion were found to be 1270 cm−1 (15.2 kJ mol−1) and 550 cm−1 (6.6 kJ mol−1), respectively. The conformational changes caused by S1←S0 electronic excitation in the fluoral molecule are similar to those observed in acetaldehyde and biacetyl molecules and differ from the conformational behavior of hexafluorobiacetyl molecule.

An Interpretation for the First- and Second-Order Phase Transitions in Nylon 66 Crystal Based on the Intra- and Intermolecular Energy Calculation

Intra- and intermolecular interactions in nylon 66 crystal are calculated mainly using internal rotation and Lennard-Jones potential functions reasonably supposing that the amide planes are fixed and the hexamethylene moiety in the chain has an inversion center. Partition function is formularized on the basis of sum of the intra- and intermolecular interaction potentials in analogy with the Ising model on a rectangular lattice with degeneracy of interaction energy to show that the first-order phase transition occurs when the transition temperature T t is lower than the critical temperature T c and the second-order phase transition does when T t = T c , where T c only depends on the intermolecular interaction and T t on the intramolecular one mainly. The pressure effect on this phenomenon is qualitatively explained from temperature- or Δ d ( d 100 - d 110,010 )-dependence of T c and T t .

Effective potential function of internal rotation in the nonrigid approximation

The feasibility of the construction of potential functions of internal rotation in the framework of semi- and nonrigid one-dimensional models is considered. The nonrigid approach under discussion is based on using ab initio quantum mechanical calculations to obtain information on molecular geometry relaxation during internal rotation and to find a physically meaningful starting approximation of the potential. Furthermore, the theoretical potential obtained is refined according to the criterion of the best agreement between the calculated and experimental frequencies of the torsional vibrations. During this latter optimization, some additional conditions may be imposed which support the maximal closeness of the results to the starting approximation. The refined potential is sensitive to the accuracy of determination of molecular geometrical parameters. For this reason, the quantum mechanical calculation should be performed at a rather high level of theory (accounting for electron correlation) to obtain reliable results. The comparison of semi- and nonrigid models of internal rotation is performed using nitrobenzene, fluoroacryloyl fluoride, 1,3-butadiene and acryloyl fluoride as examples.

Tautomeric and conformational stabilities of methylphosphonic dicyanide, methoxydicyanophosphine, and their isocyano analogs: an ab initio calculation

The relative energies and structural parameters of the equilibrium forms and the potential functions of internal rotation of methylphosphonic dicyanide, CH 3(O)(CN) 2, methoxydicyanophosphine, CH 3OP(CN) 2, and their isocyano analogs, CH 3P(O)(NC) 2 and CH 3OP(NC) 2, have been calculated at the RHF/6-31G∗ level. The total energy of the more stable oxo forms CH 3P(O)(CN) 2 and CH 3P(O)(NC) 2 are 10–20 kcal mol −1 lower than the energies of the aci forms CH 3OP(CN) 2 and CH 3OP(NC) 2. The relative stabilities of the cyano and isocyano isomers are almost the same in the case of the oxo forms, but for the aci forms the energies of the cyano isomers are 8 kcal mol −1 lower than those of the isocyano isomers. The potential curves for internal rotation in the aci forms are characterized by a deep minimum corresponding to the trans arrangement of the methyl group and the lone pair of electrons on the phosphorus atom. Two less pronounced minima are symmetrically situated with respect to relative maximum corresponding to the transition cis form. The potential curves of internal rotation in the oxo form possess three minima corresponding to staggered configurations of the methyl group and phosphorus atom bonds. The energy characteristics and geometrical parameters of the studied molecules are compared with known data for similar compounds.

Potential functions of internal rotation and electric parameters of atoms of ethane molecules and its halogenated derivatives

Based on the previously suggested approximate quantum mechanical description of potential functions of internal rotation and the calculation method, the regions of values of atomic charges and moments of ethane molecules and its halogenated derivatives have been found. They describe simultaneously experimental potential functions of internal rotation and dipole moments of molecules with an accuracy comparable to the experimental one. Potentials of internal rotation for some unstudied molecules of the series of halogenated derivatives of ethane have been predicted.

The microwave spectrum and potential function of nitrosoethane, CH 3CH 2NO

The potential function for internal rotation of the nitroso group in nitrosoethane, CH 3CH 2NO, has been determined. All available microwave data are fitted to the potential, V(α) = 1 2 ∑ n V n[1 − cos(nα)] where V 1 = (−15), V 2 = 63(2), V 3 = 389(1), V 4 = 135(1) and V 5 = 166(2) cm −1. Torsional fundamentals for the nitroso group (CN) have been measured by microwave relative intensity measurements for both rotamers: v t (cis) = 182(17) cm −1 and v t (gauche) = 61(19) cm −1. The energy difference between the two stable conformers has been determined to be ΔW(gauche - cis) = 2.1(4) kJ mol −1. The ground state torsional splitting for gauche-CH 3CH 2NO has been re-evaluated as ΔE ± = 153.5(4) MHz corresponding to a calculated value of the coupling constant T bc = 34.7 MHz. Accurate structural data have been used in the potential function calculations. The resulting potential is compared with those of some isoelectronic molecules.

The structure of nitrobenzene and the interpretation of the vibrational frequencies of the CNO2 moiety on the basis of ab initio calculations

Abstract The vibrational spectra of nitrobenzene and its para-d1, d5, 16O18O, 18O2 and 15N isotopic modifications are evaluated using the RHF/6-31G* ab initio harmonic force field. A rigorous interpretation of the experimental CNO2 moiety bands is carried out. Systematic deficiencies of the SCF method are effectively removed by applying scale factors optimized previously for a number of aliphatic nitro compounds. Fully optimized geometries are also reported for planar and orthogonal nitrobenzene conformations at the RHF and MP2 computational levels using the standard 6-31G* and 6-31G** basis sets. Theoretical geometries and barriers to internal rotation are compared with available experimental data. The calculations suggest that steric factors affect appreciably the structural parameters of the CNO2 fragment in the equilibrium planar conformation and consequently the potential function for internal rotation in nitrobenzene.

The laboratory spectrum of acetaldehyde at 1 millimeter (230-325 GHz)

The rotational spectrum of acetaldehyde (CH3CHO) in the frequency range 230-325 GHz has been measured in the laboratory using millimeter/submillimeter direct absorption spectroscopy. Over 250 transition frequencies are presented for this molecule for both A and E symmetry species in its ground (upsilon<SUB>t</SUB> = 0) and first excited (upsilon<SUB>t</SUB> = 1) torsional state, with experimental uncertainties of +/- 50 kHz. The data were fitted with a model involving an internal rotation potential function, which typically reproduces the measured frequencies to nu<SUB>obs</SUB> - nu<SUB>calc</SUB> less than or approximately 50 kHz for both ground and upsilon<SUB>t</SUB> = 1 state. These newly measured rest frequencies should aid in the identification of interstellar CH3CHO and in spectral line assignments for millimeter-band scans.

Methyl rotor effects on acetone Rydberg spectra. II. The 1B2(3s←n)←1A1 transition

Both a2(ν12) and b1(ν17) methyl torsion fundamentals and their overtones are found to be active in two-photon resonance multiphoton ionization jet spectra of the acetone 1B2(3s←n)←1A1 Rydberg transition. The acetone 3s Rydberg state torsional fundamental frequencies determined from fundamental and sequence band measurements are 118 cm−1 (ν′12) and 175 cm−1 (ν′17) compared to ground state values 77.8 and 124.5 cm−1, respectively. Corresponding values in fully deuterated acetone are 83 and 132 cm−1 compared to 53.4 and 96.0 cm−1 in the ground state. Our measured frequencies differ significantly (greatly so for the gearing torsional fundamental ν′17) from published values for both acetone-h6 and -d6. The 3s state 2ν′12, 2ν′17, and ν′12 + ν′17 frequencies are also measured allowing determination of excited state methyl torsion potential constants. The important increase (∼250 cm−1) in the 3s state V3 constant from its ground state value leads to a large increase in the potential barrier height to internal rotation, i.e., ≊1250 cm−1, 50% greater than the ≊800 cm−1 ground state barrier. This result is similar to the large increase in barrier height for eclipsed–eclipsed→staggered–staggered synchronous rotation recently found for the 1A2(3px←n) Rydberg state. There are, however, important differences in potential curvature between the 3s and 3px state internal rotation potential functions.

S1?S0 vibronic spectra and the vapor-state molecular structure of propanal and 2-methylpropanal

The vapor-state absorption spectra have been recorded for propanal PA and 2-methylpropanal MP with path lengths up to 120 m. The initial points in the S1←S0 electronic transitions have been identified together with various fundamental vibrational frequencies of the PA and MP conformers. They contain nonplanar aldehyde groups in the S1 states with inversion potential barriers of about 600 cm−1. The parameters of the internal-rotation potential functions in the S1 states have been determined, and the corresponding potential functions in the S0 states have been refined.

Conformational stability, barrier to internal rotation, structural parameters, ab initio calculations, and vibrational assignment of cyclopropylcarbonyl chloride

The fundamental asymmetric torsional mode of cyclopropylcarbonyl chloride, c-C 3H 5-CClO, for the more stable cis conformer (carbonyl group oriented cis to the ring) is observed at about 54 cm −1. The corresponding mode for the high energy trans form is assigned to a Q-branch series with stronger intensity beginning at 87.4 cm −1, decreasing in frequency with successive excited states. From a study of the Raman spectra of the vapor at variable temperatures, it has been determined that the cis is more stable than the trans form by 171 ± 35 cm −1 (489 ± 100 cal mol −1) and that the cis conformer remains in the solid. From a similar study of the spectrum of the liquid, it is found that the trans form is slightly more stable than the cis form by 93 ± 27 cm −1 (266 ± 77 calmol −1). From these data, the potential function for internal rotation of the asymmetric top has been determined and the potential coefficients are: V 1 = 528 ± 14, V 2 = 1769 ± 27 and V 3 = −371 ± 15 cm −1. This potential is consistent with cis to trans and trans to cis barriers of 1934 and 1779 cm −1, respectively, and a conformational energy difference of 155 ± 56 cm −1 (443 ± 160 cal mol −1). The r 0 structural parameters have been obtained for the cis conformer from a combination of ab initio calculated parameters with appropriate off-set values and the fit of the previously reported microwave rotational constants for the two naturally occurring chlorine isotopes. The conformational stability, barriers to internal rotation and fundamental vibrational frequencies, which have been determined from experimental data, are compared with those obtained from ab initio Hartree—Fock gradient calculations with the 3-21G * and 6-31G * basis sets and results obtained for some similar molecules.

Conformational studies by liquid crystal NMR and ab initio calculations: methyl nicotinate and methyl isonicotinate

Conformational properties of methyl nicotinate and methyl isonicotinate have been studied by liquid crystal 1H-NMR spectroscopy combined with ab initio calculations. The solvent used is a mixture of 80 mol.% of EBBA and 20 mol.% of MBBA.Ab initio calculations have been performed with 4-21G and MINI-4 basis sets to estimate molecular structures and the potential functions for internal rotation. Some structural parameters and the energy difference between rotational isomers have been refined by using observed dipolar coupling constants. The correlation between internal rotation and reorientational molecular motion has been taken into account according to the theory of Emsley, Luckhurst and Stockley. The parameters of the mean external potential are found to take similar values for methyl nicotinate and methyl isonicotinate. The energy difference of the two stable conformers of methyl nicotinate is in agreement with the analysis neglecting the correlation between the two motions.

Internal rotation potential function for anisole in solution: a liquid crystal NMR study

The 1 H and 13 C NMR spectra of anisole-methyl- 13 C dissolved in a nematic solvent ZLI 1115 have been analyzed. The set of dipolar couplings obtained is used to test various models for the internal rotations about the ring-O and O-CH 3 bonds. It is shown that the data are not consistent with the simplest models for this motion: a 2-fold jump about the ring-O bond between planar structures or a four-site jump between nonplanar structures, in both cases with a three-site jump model for motion about the O-CH 3 bond

The ground torsional state of acetaldehyde

New microwave measurements on the ground state of acetaldehyde have been carried out using a Fourier transform spectrometer in the region from 7 to 26 GHz (typical measurement uncertainty 4 kHz), and a conventional Stark spectrometer in the region from 45 to 116 GHz (typical measurement uncertainty 40 kHz). These new ground state measurements and remeasurements have permitted a much better fit to two theoretical models of a data set containing far-infrared combination differences from the literature, microwave transitions from the literature, and the new microwave transitions. Root-mean-square residuals obtained here for all these data (which come from a large number of sources) are only slightly larger (for either model) than the estimated measurement uncertainties. The first theoretical model is essentially a high-barrier effective Hamiltonian for one vibrational state only, based on Fourier expansions in terms of the form cos( 2πn 3 )(ρK − σ) . The second model is based on calculation using the internal-rotation potential function, and is in principle much more powerful than the first. The present successful fits using either model indicate that earlier fitting difficulties using the second model and a combined infrared and microwave data set were caused by problems in the microwave data set, rather than problems in the model. It is hoped that similar success can be achieved with the more powerful second model when data from higher excited torsional states are considered.