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1.
The microwave spectra of the ground and five excited states of a second gauche rotamer of allylamine have been measured and assigned. Three of the excited states belong to the same mode, most probably the CC torsion, the second and third vibrational states present a symmetrical splitting due to tunneling effect. The spectrum was conclusively identified as due to the N-gauche, lone-electron-pair trans form by means of the N-quadrupole coupling constants and dipole moment components. The variation observed for the quadrupole coupling constants in the different vibrationally excited states was explained by a suitable model. The ground state constants are (in MHz) A0 = 23 957.05 ± 0.048, B0 = 4 229.96 ± 0.025, C0 = 4 154.91 ± 0.025, χaa = ? 1.48 ± 0.04, χbb - χcc = ? 1.42 ± 0.04, and (in D) ∥μa∥ = 0.766 ± 0.010, ∥μb∥ = 0.700 ± 0.005, ∥μc∥ = 0.290 ± 0.020.The excited states of the N-cis, lone-electron-pair trans form were also measured and assigned; two of these states appear to belong to the CC torsion as indicated by their intertial defects. The potential hindering the internal CC rotation was calculated using the relative intensity data of the N-cis and N-gauche forms as well as the tunneling splittings. A three-term cosine potential was fitted to the data yielding (in cm?1) V1 = ? 77 ± 85, V2 = 170 ± 126, V3 = 663 ± 95. The Dennison-Uhlenbeck potential was used for an approximate calculation of the N-trans barrier separating the two identical N-gauche forms. The barrier obtained was 1.9 ± 0.3 Kcal/mole. 相似文献
2.
The microwave spectrum of N-cyanopyrrolidine was observed and assigned in the ground and nine excited states. In the lowest two states, split 3.9 cm?1 by a ring-puckering, nitrogen-inversion motion, the rotational constants are (for v = 0) A = 6585.05 ± 3.83, B = 1919.54 ± 0.05, C = 1583.84 ± 0.05, and (for v = 1) A = 6575.31 ± 6.01, B = 1922.37 ± 0.08, C = 1586.44 ± 0.08 MHz. Deviations from rigid rotor behavior in the lowest two states were described and analyzed by inclusion of a Hamiltonian term coupling the states via the internal vibrational angular momentum. The observed conformation of the five-membered ring system was found to be the envelope equatorial form. The tunneling motion which interconverts equivalent conformers has been discussed, and the qualitative nature of the potential energy surface has been described and compared to the parent unsubstituted molecule. 相似文献
3.
The microwave spectrum of N-acetylglycine was obtained using a NIST Fourier-transform microwave spectrometer equipped with a heated, pulsed-nozzle source. One conformer has been identified and its spectrum assigned. The conformer has CS point group symmetry and an intramolecular hydrogen bond between the carbonyl and amide groups of the 5-membered glycine unit. Internal rotation of the methyl rotor group leads to splitting of the rotational lines into A and E symmetry tunneling states. The 14N nuclear-quadrupole hyperfine structure verifies the rotational and internal-rotor state assignments. The V3 barrier of 57.5(1) cm−1 and the angles between the C3 axis of the methyl rotor and the principal inertial axes are in best agreement with the calculated values for the lowest energy conformer of the four conformers predicted at the MP2/6-311++G(d,p) level of theory. 相似文献
4.
The centrifugal distortion analysis of the microwave spectrum of the N-gauche lone-electron-pair trans (NGLT) rotameric form of allylamine has been carried out in the frequency region 5–40 GHz and up to J = 29 in its ground vibrational state. The analysis gives effective rotational constants and all the quartic centrifugal distortion constants. 相似文献
5.
The jet-cooled Fourier-transform microwave spectrum of N,N-dimethylacetamide was recorded in the region of 12-24 GHz, and was analyzed to determine rotational constants and nuclear quadrupole coupling constants. Coriolis-like coupling parameters characterizing interaction between internal rotation of methyl groups and the overall rotation were also determined from internal-rotation tunneling splittings of the rotational transitions. The Coriolis-like coupling parameters permitted determination of the barrier heights to internal rotation of the three methyl groups, which were found to be 677, 237, and 183 cm−1 for the C-methyl top, the trans-N-methyl top and the cis-N-methyl top, respectively. 相似文献
6.
The jet-cooled Fourier-transform microwave spectrum of N-methylacetamide (CH3NHC(O)CH3), a molecule containing two methyl tops with relatively low barriers to internal rotation, has been recorded and fit to nearly experimental uncertainty. Measurements were carried out between 10 and 26 GHz, with the nitrogen quadrupole splittings resolved for many transitions. The permutation-inversion group for this molecule is G18 (not isomorphic to any point group), with irreducible representations A1, A2, E1, E2, E3, and E4. One of these symmetry species and the usual three asymmetric rotor quantum numbers JKaKc were assigned to each torsion-rotation level involved in the observed transitions. F values were assigned to hyperfine components, where . Transitions involving levels of A1 and A2 species could be fit to an asymmetric rotor Hamiltonian. The other transitions were first fit separately for each symmetry species using a Pickett-like effective rotational Hamiltonian. Constants from these fits show a number of additive properties which can be correlated with sums and differences of effects involving the two tops. A final global fit to 48 molecular parameters for 839 hyperfine components of 216 torsion-rotation transitions involving 152 torsion-rotation levels was carried out using a newly written two-top computer program, giving a root-mean-square deviation of observed-minus-calculated residuals of 4 kHz. This program was written in the principal axis system of the molecule and uses a free-rotor basis set for each top, a symmetric-top basis set for the rotational functions, and a single-step diagonalization procedure. Such an approach requires quite long computation times, but it is much less prone to subtle programming errors (a consideration felt to be important since checking the new program against precise fits of low-barrier two-top molecules in the literature was not possible). The two internal rotation angles in this molecule correspond to the Ramachandran angles ψ and φ often defined to describe polypeptide folding. Barriers to internal rotation about these two angles were found to be 73 and 79 cm−1, respectively. Top-top coupling in both the kinetic and potential energy part of the Hamiltonian is relatively small in this molecule. 相似文献
7.
The microwave spectrum of normal trans-ethylamine CH3CH2NH2 and that of the -NHD and -ND2 species were measured and assigned. The obtained rotational constants for the ground state of the normal species are (in MHz): A = 31 758.33 ± 0.08, B = 8749.157 ± 0.025, and C = 7798.905 ± 0.025. The fitted dipole moment components are (in Debye): |μ|a = 1.057 ± 0.006, |μb| = 0.764 ± 0.009, and |μt| = 1.304 ± 0.011. The quadrupole coupling constants were fitted as (in MHz): χ+ = 1.62 ± 0.035 and χ? = ?1.89 ± 0.08. Analysis of the HFS of the deuterated species -ND2 allowed the experimental determination of the principal quadrupole tensor values (in MHz): χzz = ?4.68 ± 0.20, χyy = 1.75 ± 0.06, and χxx = 2.93 ± 0.20. The angle between the CN bond and the direction of the χzz quadrupole tensor component was fitted as 108.9° ± 0.6° and agreed with the expected general direction of the lone electron pair. 相似文献
8.
The microwave spectrum of the ground state of the normal species of gauche-ethylamine CH3CH2NH2 and that of -NHD, -NDH, as well as -ND2 isotopic species were measured and assigned. The ground state splits into four substates due to two internal large amplitude motions: inversion (s and a) and internal rotation (o and e) about the CN axis. Intersystem transitions due to tunneling as well as vibrational-rotational perturbations affect not only the absorption frequencies but also the Stark effect and NQHFS. The rotational constants for the two symmetrical inversion states (s) were fitted for the normal species as (all values in MHz) Ase = 32 423.470 ± 0.184, Bse = 8 942.086 ± 0.039, and Cse = 7 825.520 ± 0.048, and Aso = 32 378.733 ± 0.182, Bso = 8 940.906 ± 0.052, and Cso = 7 825.551 ± 0.042 with the interaction constants Qas = 151.12 ± 0.52 and Qbs = 44.4 ± 7.0. The antisymmetrical inversion states (a) were fitted as Aae = 32 423.347 ± 0.142, Bae = 8 942.027 ± 0.029, and Cae = 7 825.525 ± 0.031, and Aao = 32 378.720 ± 0.142, Bao = 8 940.984 ± 0.029, and Cao = 7 825.573 ± 0.031 with the interaction constants Qaa = 167.10 ± 0.31, Qba = 48.1 ± 5.4. The energy splitting due to intersion was determined (in MHz) as Δνinv = 1 391.39 ± 0.19 and that due to internal rotation as Δνtors = 1 170.58 ± 0.18. The cis barrier separating the two equivalent torsional states was calculated as 690 cm?1, and the inversion barrier between the inversion states was calculated as 1400 cm?1, both using the Dennison-Uhlenbeck model. A simple model explaining the inversion splittings of the monodeuterated species is proposed. Comparing the relative intensities for several temperatures the gauche form was observed to be energetically higher than the trans form by 110 ± 50 cm?1. The dipole moment could only be fitted by taking into account the internal motions yielding (in Debye) μaeff = 0.11 ± 0.01, μbeff = 0.65 ± 0.01, and μceff = 1.014 ± 0.015. The quadrupole coupling constants (in MHz) were found as χaa = ?χ+ = 2.268 ± 0.043 and χbb ? χcc = χ? = 3.120 ± 0.035. 相似文献
9.
Masaharu Fujitake Junpei Aoyama Nobukimi Ohashi 《Journal of Molecular Spectroscopy》2006,235(1):27-34
The jet-cooled Fourier-transform microwave spectrum of N-methylaniline (C6H5-NHCH3) was recorded in the region of 10-26 GHz, and was analyzed to determine rotational constants and nuclear quadrupole coupling constants. Furthermore, a Coriolis-like coupling parameter characterizing an interaction between an internal rotation of a methyl group and an overall rotation was also determined from A-E splittings observed in pure rotational transitions with high Ka values. The Coriolis-like coupling parameter permitted the determination of the barrier to internal rotation of the methyl group which was found to be 975 cm−1. 相似文献
10.
Microwave spectra of the anti rotamer of vinyl mercaptan and its SD isotopic species have been studied in the frequency range 12–60 GHz. For the normal species rotational and centrifugal distortion constants have been obtained for the ground and first three excited states of the SH torsional mode, the ground state values being A = 49 422.75(5) MHz, B = 5 897.215(9) MHz, C = 5 279.436(9) MHz, DJ = 3.12(11) kHz, DJK = ?38.50(1.71) kHz, and δJ = 0.498(51) kHz. An approximate potential function for the SH torsion in the vicinity of the anti conformation, derived using the observed variation of rotational constants with vibrational quantum number, reveals the presence of a small potential barrier of 19 cm?1 at the planar conformation. The v = 0 state lies above this barrier so the molecule is essentially planar in the ground state in spite of the observed negative value for the inertia defect (?0.1976(2) a.m.u.Å2). The anti rotamer is found to be 50 ± 25 cm?1 less stable than the syn rotamer. The dipole moment has the ground state values μa = 0.425(10), μb = 1.033(10), and μtotal = 1.117(14) D and is shown to vary considerably with vibrational quantum number. Evidence for significant structural changes in going from the syn rotamer to the anti rotamer is also presented. 相似文献
11.
Mitsutoshi Tanimoto V. Almond S.W. Charles J.N. Macdonald N.L. Owen 《Journal of Molecular Spectroscopy》1979,78(1):95-105
Rotational spectra of vinyl mercaptan (ethenethiol) CH2CHSH and its isotopic modification CH2CHSD have been studied by microwave spectroscpy. The molecule has been found to exist in two rotameric forms, syn and anti, associated with different orientations of the SH bond with respect to the vinyl framework. In this paper results are reported for the more stable syn form which is shown to be planar with ground state rotational constants A = 49 815.28(6) MHz, B = 5835.716(14) MHz, C = 5222.081(11) MHz, DJ = 2.85(17) kHz, DJK = ?33.22(2.08) kHz, and δJ = 0.425(65) kHz. Spectra have also been observed for the first and second excited states of the SH torsional vibration and the first excited state of the CCS angle bending mode. The dipole moment of the syn rotamer is μa = 0.813(1), μb = 0.376(4), and μtotal = 0.896(3) D. 相似文献
12.
P.D. Mallinson 《Journal of Molecular Spectroscopy》1977,68(1):68-76
The ground vibrational state microwave spectrum of CHD2Cl has been studied in the region 26.5–40.0 GHz. From the observation of weak c-type transitions the A0 rotational constants of CHD235Cl and CHD237Cl have been determined to be 95 426.08 ± 0.06 and 95 425.23 ± 0.11 MHz, respectively. The observed a-type and c-type transitions have been used to obtain A, B, C, all five quartic and one sextic distortion constants present in the reduced Hamiltonian of Watson for the 35Cl and 37Cl isotopic modifications of CHD2Cl. 相似文献
13.
B.P. van Eijck 《Journal of Molecular Spectroscopy》1981,85(1):189-204
The microwave spectrum of cis-2-pentene has been shown to originate from molecules in the skew-conformation (dihedral angle φ = 119 ± 3°). From A-E doublet splittings the barrier to internal rotation about the C1C2 axis was found to be 280 ± 4 cm?1; furthermore the dipole moment components and centrifugal distortion constants are reported. In excited states of the C3C4 torsion the spectra exhibit further splittings; these are due to tunneling between the two equivalent skew-conformations through a barrier of 210 ± 20 cm?1. 相似文献
14.
The microwave spectrum of MnO3F has been remeasured and several corrections and new results have been obtained: B0 = 4129.141 MHz, DJ = 1.12 kHz, DJK = 1.87 kHz; , , , |q5| = 16.005, and |q6| = 8.456 MHz. 相似文献
15.
Robert G. Ford 《Journal of Molecular Spectroscopy》1974,49(1):117-123
Rotational transitions of 1-nitropropene arising from the ground vibrational state and from three excited states of the nitro torsional vibration have been assigned. The values of the rotational constants in MHz are: A0=10 650B0=2028.56C01722.16A110 615 B1=2028.47 C1=1725.11 A210 570 B2 2028.31 C2= 1727.32 A3= 10 512 B32028.11 C3=1729.37The dipole moment components are μa = 4.52 D, μb = 0.42 D and μtotal = 4.54 D. From the lack of observable internal rotation splittings the barrier to internal rotation of the methyl group is shown to be greater than 2250 cal/mole. 相似文献
16.
The microwave spectrum of 2,8,9-trioxaadamantane has been investigated in the region from 12.4 to 26.5 GHz. The observed spectrum exhibited the expected symmetric top pattern, with the rotational constant B0 = 1848.64 MHz. Numerous weaker lines were observed and were attributed to vibrational satellites of the main rotational transition. The transitions from J = 3 → 4 through J = 6 → 7 were studied and no centrifugal distortion effects were observed.A structure is derived that is consistent with the observed rotational constants of the normal and one isotopic species by use of the method of diagnostic-least-squares.The second order stark effect for the K = 0 state yielded a dipole moment of 3.01 ± 0.03 D. 相似文献
17.
Microwave spectra of tetrazole and the two singly deuterated tetrazole species have been assigned. Dipole moment measurements suggest that the tautomeric form observed in normal tetrazole and N-deuteriotetrazole is 2H-tetrazole while that observed in C-deuteriotetrazole is 1H-tetrazole. All three species are planar. 相似文献
18.
19.
The J = 101-000 and 202-101 transitions of nine isotopomers of chlorogermylene, H74Ge35Cl, H74Ge37Cl, H72Ge35Cl, H72Ge37Cl, H70Ge35Cl, H70Ge37Cl, H76Ge35Cl, H76Ge37Cl, and H73Ge35Cl are measured at 8-9 and 16-18 GHz. The effective rotational constants, the nuclear quadrupole coupling constants of 35Cl, 37Cl, and 73Ge, and the nuclear spin-rotation coupling constants of 35Cl and 37Cl are determined. 相似文献
20.
The microwave spectrum of urea has been recorded between 5 and 50 GHz. The rotational constants obtained from a least-squares fit of 47 b-type transitions are A = 11233.333, B = 10369.369, C = 5416.668 MHz. Centrifugal distortion constants were evaluated subject to the planarity constraints, giving satisfactory agreement up to at least J = 20. They are: τaaaa = ?0.04423, τbbbb = ?0.04075, τaabb = 0.02268, τabab = ?0.02806. The diagonal elements of the nuclear quadrupole coupling tensor are χaa = 2.16, χbb = 1.88, χcc = ?4.04. The dipole moment is μ = μb = 3.83 D (12.8 × 10?30 Cm). 相似文献