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1.
The rotational spectra of two isotopologues of the 1:1 complex between chlorotrifluoromethane and formaldehyde have been recorded and analyzed by using Fourier‐transform microwave spectroscopy. Only one rotamer was detected, with the two constituent molecules held together through a Cl???O halogen bond (RCl???O=3.048 Å). The dimer displays two simultaneous large‐amplitude intramolecular motions. The internal rotation of formaldehyde around its symmetry axis (V2=28(5) cm?1) splits all the rotational transitions into two component lines with a relative intensity ratio of 1:3. On the other hand, the almost free internal rotation (V3≈2.5 cm?1) of the CF3 symmetric top increases the “rigid” value of the rotational constant A by almost one order of magnitude. In addition, all the transitions display a hyperfine structure due to the 35Cl (or 37Cl) nucleus quadrupole effects.  相似文献   

2.
《Chemical physics letters》1986,132(2):103-107
The torsional data for CF3NO have been rein vest igated. A model with a single degree of freedom and three adjustable parameters is sufficient to fit data to v = 8 in the electronic ground state. For CF3NO we obtain Fo = 1.9822(42) cm−1, V3 = 238.4(1.6) cm−1 and V6 = −5.8(1.6) cm−1 or Fo = 1.9894(66) cm−1,F3= −0.194(55) cm−1 and V3 = 239.3(1.9) cm−1. A similar treatment for CF3CHO gives Fo = 1.97(14) cm−1, V3 = 305(25) cm−1 and V6 = −8.7(1.2) cm−1. A need for a re-examination of the torsional fundamental is indicated for CF3CHO. These studies support the general conclusion that for a heavy internal top the internal rotation constant, Fo, required to fit a range of torsional splittings is different from that calculated from structural considerations alone. The difference indicates a large change in F with torsional averaging.  相似文献   

3.
The Fourier transform microwave spectra of the E and Z isomers of butadienyl acetate were measured in the frequency range from 2 to 26.5 GHz under molecular‐jet conditions. The most stable conformer of each isomer, in which all heavy atoms are located in a symmetry plane, was identified after analyzing the spectrum by comparison with the results from quantum‐chemical calculations. The barriers to internal rotation of the acetyl methyl group were found to be 149.1822(20) and 150.2128(48) cm?1 for the E and Z isomers, respectively, which are similar to that of vinyl acetate. A comparison between two theoretical approaches treating internal rotation, the rho axis method and combined axis method, was also performed. The influence of the alkyl R chain on the methyl torsional barriers in CH3 ‐COOR acetates was explored.  相似文献   

4.
The methods for analyzing the vibrational structure of high-resolution UV spectra and long-wave IR Fourier transform spectra in studies of internal rotation in α,β-unsaturated carbonyl compounds R4R3C=CR2-COR1 (R1 = F, Cl; R2 = R3 = R4 = H, CH3) are compared. These methods were found to give different experimental values for systems of torsional vibration energy levels up to high quantum numbers, torsional frequencies (0–1 transitions), and anharmonicity coefficients x 11 for trans and cis isomers of the same molecules in the ground electronic state (S 0). It was shown that the experimental technique for analyzing the vibrational structure of UV spectra excludes the hydrolysis of compounds under study. Taking into account Fermi resonance and numerous Deslandres tables constructed for trans and cis isomers provides reliable determination of values necessary for the construction of internal rotation potential functions, because they are multiply repeated in various Deslandres tables. An analysis of the vibrational structure of UV spectra gives more reliable V n internal rotation potential function parameters. The V n parameter values were substantiated by quantum-mechanical calculations performed by other authors.  相似文献   

5.
The systems of torsional vibration levels of the trans and cis methacryloyl chloride isomers in the ground (S 0) and excited (S 1) electronic states obtained by analyzing the vibrational structure of the gas-phase UV spectrum were used to reproduce the internal rotation potential functions of the molecule in both electronic states. The kinematic F factor in the S 0 and S 1 electronic states was calculated taking into account the relaxation of geometric parameters depending on the internal rotation angle. The internal rotation potential function parameters in the S 0 state are substantially different from the parameters obtained using the torsional levels of the IR Fourier transform spectrum; at the same time, they are substantiated by quantum-mechanical calculations.  相似文献   

6.
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.  相似文献   

7.
Using two molecular jet Fourier transform spectrometers, the microwave spectrum of hexan-2-one, also called methyl n-butyl ketone, was recorded in the frequency range from 2 to 40 GHz. Three conformers were assigned and fine splittings caused by the internal rotations of the two terminal methyl groups were analyzed. For the acetyl methyl group CH3 COC3H6CH3, the torsional barrier is 186.9198(50) cm−1, 233.5913(97) cm−1, and 182.2481(25) cm−1 for the three observed conformers, respectively. The value of this parameter could be linked to the structure of the individual conformer, which enabled us to create a rule for predicting the barrier height of the acetyl methyl torsion in ketones. The very small splittings arising from the internal rotation of the butyl methyl group CH3COC3H6 CH3 could be resolved as well, yielding the respective torsional barriers of 979.99(88) cm−1, 1016.30(77) cm−1, and 961.9(32) cm−1.  相似文献   

8.
The microwave spectrum of propionyl chloride has been investigated in the region 18.0–40.0 GHz, and transitions due to a cis conformer have been assigned. This form has a heavy atom planar configuration and the methyl group and the carbonyl oxygen atom are cis to each other. Using the substitution structures of propionic acid and acetyl chloride as molecular models for the propionyl chloride molecule, good agreement is found between observed and calculateò effective rotational constants. For the 35Cl species satellite spectra assigned to the first four excited states of the C-C torsional mode have been observed together with the first excited state of the methyl torsional mode. The ground state spectrum has also been assigned for the 37Cl species. Relative intensity measurements yielded the lowest C-C torsional vibration frequency of 86 ± 10 cm?1. The CH3 internal rotation frequency was found to be 197 cm?1. Nuclear quadrupole coupling constants were determined for the ground state of the 35Cl and 37Cl species. From observed A-E splittings of bQ-branch transitions of the first excited state of the methyl torsional mode a barrier to internal rotation was estimated to be V3 = 2480 ± 40 cal mol?1 (867 ± 14 cm?1).  相似文献   

9.
The structure of the conformationally nonrigid fluoral molecule (CF3CHO) in the ground (S0) and lowest excited triplet (T1) and singlet (S1) electronic states was studied by ab initio quantum-chemical methods. The equilibrium geometric parameters and harmonic vibrational frequencies of the molecule in these electronic states were determined. The calculations demonstrated that the electronic excitation causes substantial changes in the molecular structure involving the rotation of the CF3 top and the deviation of the CCHO carbonyl fragment from planarity. The quantum-mechanical problems for large-amplitude vibrations, namely, for the torsional vibration in the S0 state and the torsional and inversion vibrations (nonplanar carbonyl fragment) in the T1 and S1 states, were solved in the one- and two-dimensional approximations. A comparison of the results of calculations revealed the correlation between the torsional and inversion motions.  相似文献   

10.
Jacobsen  S.  Andresen  U.  Mäder  H. 《Structural chemistry》2003,14(2):217-225
The rotational spectra of o-fluorotoluene and its seven 13C isotopic species were recorded in the frequency range from 4 to 20 GHz with employment of pulsed molecular beam Fourier-transform microwave (MB-FTMW) spectrometers. The analysis of the spectra in the two lowest states of methyl internal rotation (torsional ground state, A and E species) was based on a asymmetric frame-rigid symmetric top Hamiltonian with inclusion of centrifugal distortion terms, yielding structural rotational constants, as well as the threefold barrier V 3 to internal rotation and the angle(a,i) between the principal moment of inertia a axis and the internal rotor axis i. The rotational constants of all eight isotopomeres were used to derive the seven 13C r s coordinates of the molecule.  相似文献   

11.
The results of the analysis of the fluorescence excitation spectrum of trans-hexafluorobiacetyl published earlier are revised. New values of torsion frequencies and potential barriers of internal rotation of the CF3 group in the ground and excited states of the molecule are obtained. A procedure for calculating the probabilities of torsional vibronic transitions of molecules is described. Translated fromZhurnal Struktumoi Khirnii, Vol. 38, No. 2, pp. 293–302, March–April, 1997.  相似文献   

12.
The far infrared spectrum (375 to 30 cm–1) of gaseous 2-chloro-3-fluoropropene, CH2=C(CH2F)CI, has been recorded at a resolution of 0.10 cm–1. The fundamental asymmetric torsional mode is observed at 117.5 cm–1 with ten excited states falling to low frequency for thes-cis (fluorine atom eclipsing the double bond) conformer. For the higher energy gauche conformer, the asymmetric torsion is estimated to be at 94 cm–1. From these data the asymmetric torsional potential function has been calculated. The potential function coefficients are calculated to be in cm–1):V 1=803±21,V 2=–94±21,V 3= 1025±10,V 4=95±10, andV 6=2±1, with an enthalpy difference between the more stables-cis and gauche conformera of 550±100 cm–1 (1.57±0.29 kcal/mol). This function gives values of 1227±50cm–1(3.51±0.14kcal/mol), 1266±200 cm–1 (3.62±0.57 kcal/mol), and 665±100 cm–1 (1.90±0.29 kcal/mol), for thes-cis to gauche, gauche to gauche, and gauche tos-cis barriers, respectively. From the relative intensities of the Raman lines of the gas at 652 cm–1 (gauche) and 731 cm–1 (s-cis) as a function temperature, the enthalpy difference is found to be 565±96 cm–1 (1.62±0.27 kcal/mol). However, the more polar gauche conformer remains in the crystalline solid. The Raman spectrum of the gas has been recorded from 3500 to 70 cm–1 and, utilizing these data and the previously reported infrared data, a complete vibrational analysis is proposed for both conformers. The conformational stability, barriers to internal rotation, fundamental vibrational frequencies, and structural parameters that have been determined experimentally are compared to those obtained from ab initio Hartree-Fock gradient calculations employing both the 3–21 G* and 6–31G* basis sets and to the corresponding quantities for some similar molecules.  相似文献   

13.
The microwave spectrum of isobutene has been recorded from 10 to 35 GHz. From the analysis of the ground and first two excited torsional state splittings, the following internal rotation parameters were calculated: V3 = 2170 cal mol.?1, V'12 = ?210 cal. mol.?1, Iα = 3.18amu Å2 and angle (methyl-top to b-axis) 58.21°. Centrifugal distortion parameters were also obtained for the ground state.  相似文献   

14.
The rotational spectra of several torsional satellites of methyl glycolate and 2-methoxyethanol have been investigated.The methyl barrier to internal rotation in methyl glycolate increases with the torsional quantum number of the C-C skeletal torsion, for which A–E line splittings have been measured up to vSK=3. The V3 value determined from the A–E line splittings in the first excited state of the methyl internal rotation is nearly the same of that previously determined in the ground state.For 2-methoxyethanol the V3 barrier as determined from the A–E line splittings in the ground state is about 20% lower than the value previously obtained from the splittings observed in the first excited state of the methyl internal rotation. The sequence of the A–E splittings in the first excited state of the O-C skeletal torsion (vSK=1) is probably reversed with respect to the ground state, while in the vSK=2 state the sequence is like that in the ground state.  相似文献   

15.
16.
The potential energy profile of an isolated CF2ClCOOH molecule with a CF2Cl group rotating around the C–C bond was determined by the Hartree–Fock method using the 6-31G(d) basis set. Barriers to internal rotation were estimated for this molecule; its geometrical parameters were found for the equilibrium and transition states that are due to the torsion potential with unequal wells. Crystal effect on CF2Cl reorientations in solid chlorodifluoroacetic acid has been evaluated.  相似文献   

17.
The infrared spectra (3200 to 30 cm–1) of gaseous and solid chloroacetyl bromide, CH2ClC(O)Br, and the Raman spectra (3200 to 10 cm–1) of the gas, liquid (with depolarization data), and solid have been recorded. From the observed asymmetric torsional transitions, the potential function governing internal rotation of the CH2Cl moiety has been determined with the following coefficients:V 1=336±11,V 2=73±10,V 3=757+7,V 4=103±3, andV 6=5±2 cm–1. This potential function is consistent with s-trans to gauche and gauche to gauche barriers of 963±11 and 709±12cm–1, respectively, and enthalpy difference of 373 ± 24 cm–1 with the dihedral angle of the gauche rotamer being 115°. The enthalpy difference has been determined experimentally from the studies of the Raman spectra at different temperatures to be 359±68 cm–1 (1.03±0.19 kcal mol–1) and 507±24 cm–1 (1.45±0.07 kcal mol–1) for the gas and liquid, respectively, with the s-trans conformer being the more stable conformer in the gas and liquid and the only one present in the annealed solid. A complete assignment of the vibrational fundamentals is proposed from spectral data obtained for the gas, liquid, and solid. The assignment is supported by a normal coordinate calculation utilizing a modified valence force field to obtain the frequencies for the normal vibrations and the potential energy distribution. The results are discussed and compared to the corresponding quantities for some similar molecules.Taken in part from the thesis of H. V. Phan, which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree.  相似文献   

18.
One of the major goals of conformational analysis is the calculation of the energy difference between two or more conformers, ΔE, as well as the energy necessary for interconversion. The calculation of these energy quantities is facilitated by using a potential function which describes the vibrational motion, or internal rotation (torsion), as a function of the dihedral angle, α. The potential function is called asymmetric because both the frame and top portions of the molecule have no symmetry element higher than a plane. The most common type of potential function where at least one of the minima coincides with the plane of symmetry is of the type: V(α) = 12ΣVi(1 - cos iα). The kinetic energy term, F(α), is extremely complicated. In general, if the only data being used to calculate the potential function are torsional transitions, and if one continues within the boundary conditions of a one-dimensional problem, then a cosine expansion of F(α) should be adequate: F(α) = F0 + ΣFi cos iα. For those systems where there is an equilibrium between a planar form and two non-planar forms, V3 is usually the predominant term. This is because V3 represents a three maxima/three minima potential per 2π (360°) internal rotation. In a similar fashion, V2 is found to be the predominant term in the potential function for a system consisting of two equivalent non-planar conformers. Several examples of our most recent studies are given where the potential function for interconversion of two conformers has been determined.  相似文献   

19.
The gas‐phase structures and parameters describing acetyl methyl torsion of N‐ethylacetamide are determined with high accuracy, using a combination of molecular beam Fourier‐transform microwave spectroscopy and quantum chemical calculations. Conformational studies at the MP2 level of theory yield four minima on the energy surface. The most energetically favorable conformer, which possesses C1 symmetry, is assigned. Due to the torsional barrier of 73.4782(1) cm?1 of the acetyl methyl group, fine splitting up to 4.9 GHz is found in the spectrum. The conformational structure is not only confirmed by the rotational constants, but also by the orientation of the internal rotor. The 14N quadrupole hyperfine splittings are analyzed and the deduced coupling constants are compared with the calculated values.  相似文献   

20.
Internal rotation in the C·H2(CH2) n NO2 (n ≤ 7) type radicals has been studied. 44 potential functions of the internal rotation, V(φ), have been calculated taking advantage of the B3LYP/6-311++(3df,3pd) and MP2/6-311++(3df,3pd) methods. The trends observed in the series of parameters characterizing the internal rotation have been explained in view of the electron clouds conjugation, the inductive effect of the end groups, the gauche effect, and the rotation tops interaction. The coefficients of V(φ) have been shown to depend predominantly on the nearest surrounding of the rotation axis. Based on this, the generalized functions, V av(φ), have been developed, their coefficients being dependent exclusively on the rotating bond position. Such functions are convenient for molecular modeling applications.  相似文献   

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