I. Botskor

The microwave spectrum of gauche-ethylamine

Abstract 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.

The microwave spectrum of trans-ethylamine

Abstract The microwave spectrum of normal trans -ethylamine CH 3 CH 2 NH 2 and that of the -NHD and -ND 2 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, | μ b | = 0.764 ± 0.009, and | μ t | = 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 -ND 2 allowed the experimental determination of the principal quadrupole tensor values (in MHz): χ zz = −4.68 ± 0.20, χ yy = 1.75 ± 0.06, and χ xx = 2.93 ± 0.20. The angle between the CN bond and the direction of the χ zz quadrupole tensor component was fitted as 108.9° ± 0.6° and agreed with the expected general direction of the lone electron pair.

The microwave spectrum of the fourth distinct rotameric form of allylamine: The N-cis lone-electron-pair gauche conformation

Abstract The microwave spectrum of the ground state of the N-cis lone-electron-pair gauche rotamer (NCLG) of allylamine (CH2CHCH2NH2) has been measured and assigned. The rotamer appears in two geometrically equivalent forms which are symmetrical with respect to the heavy atom plane. The low barrier restricting torsion about the CN bond removes the degeneracy and the ground state appears split into the symmetrical 0+ and the antisymmetrical 0− states. Through the explicit fitting of rovibrational frequencies connecting levels of different parity, the 0+ and 0− states were found to be separated by Δνtors = 4574.98 ± 0.093 MHz and having the rotational constants (in megahertz): A 0 + = 16 308.87 ± 0.022 ; B 0 + = 5799.50 ± 0.015 ; C 0 + = 4458.10 ± 0.015 ; A 0 − = 16 304.17 ± 0.022 ; B 0 − = 5796.20 ± 0.014 ; C 0 − = 4458.20 ± 0.012 . Two more (interaction) constants could also be determined: Q a = 108.98 ± 0.13 and Q b = 94.9 ± 0.8 . The quadrupole coupling constants were found as χaa = +1.05 ± 0.06 and χbb − χcc = 1.81 ± 0.16. The dipole moment components could be analyzed only by taking into explicit consideration a suitable approximation for the tunneling effect (quantities in debye): μ a eff = 0.804 ± 0.003 ; μ b eff = 0.78 ± 0.01 ; μ c eff = 0.78 ± 0.01 .

Microwave spectrum of 3-Fluoropropyne

Abstract The microwave spectra of the normal and six isotopic species of propargyl fluoride, HCCCH2F, have been measured and their rotational constants have been fitted. The rs structure was determined as CH(methine) = 1.056(2) A, CC = 1.206(4) A, CC = 1.454(4) A, CH(methyl) = 1.096(2) A, HCC = 179.6(7)°, CCC = 178.9(8)°, CCH = 110.6(2)°, and HCH = 109.3(2)°. The center of mass conditions were used to calculate the position of the fluorine atom. The structural parameters derived are CF = 1.393(6) A and CCF = 111.0(4)°. The errors stated were calculated from “Costains rule,” those due to the uncertainties of the rotational constants are an order of magnitude smaller. The carbon chain is slightly bent away from the fluorine atom within the plane defined by the CC and CF bonds. There is no significant HCC bend and no significant methyl group tilt. The dipole moment was fitted (in Debye) as μa = 1.03 ± 0.01, μb = 1.40 ± 0.01, and μtot = 1.73 ± 0.02. The microwave spectra of four low-lying excited vibrational states have also been assigned, and their rotational constants and centrifugal distortion constants have been determined.

The application of laser-microwave double and triple resonance to a complex molecular system: Allylamine

Abstract Information relevant to the infrared spectrum and to the tunneling barrier in highly excited states has been obtained for allylamine from a series of laser-microwave double- and triple-resonance experiments. An extremely large number of double-resonance signals were observed in this molecule with the 10.8-μm, R (8), 13 CO 2 laser line. From the extensive use of triple-resonance experiments this laser line was shown to be coincident with infrared transitions in two distinct rotameric forms of allylamine. In the two infrared transition systems analyzed the upper vibrational states proved to be badly perturbed. The use of the triple-resonance technique as an assignment aid under such circumstances is discussed. Rotational constants for two highly excited vibrational states of allylamine were obtained and in one case evidence is presented for a possible large effective lowering of the tunneling barrier. The center frequencies of the two vibrational bands concerned are calculated.

Barrier to internal rotation of the silyl group in cyclopropyl silane from the MW spectrum in the first excited torsional state

Abstract The MW spectrum of the first excited state of the internal rotation of the silyl group in cyclopropyl silane has been recorded and analyzed in the frequency region 6.9–38.0 GHz. Recordings were made with conventional Stark spectroscopy as well as with MW-MW double resonance. From the analysis of the torsional splittings the following parameters were derived: V 3 =1917.6(44) cal/mole , I α =5.888(14) u ∗ A 2 , V a =21.56(21)°

Heavy atom structure of the NCLT rotamer of allylamine

Abstract The rotational spectra of the three singly 13 C-substituted isotopic species of the N cis , lone pair trans (NCLT) rotamer of allylamine have been measured in natural abundance by double resonance modulated microwave spectroscopy. For the previously studied normal and ND 2 species additional very weak μ b lines have been measured in order to improve the precision of the A rotational constants. Assuming reasonable values for the geometry of the carbon-hydrogen bonds, the following bond parameters of the heavy atom skeleton of allylamine-NCLT are derived from a least-squares fit to the differences of the moments of inertia between the normal (parent) form and five isotopic species: r (CC) = 1.339(5), r (CC) = 1.499(11), r (CN) = 1.456(11), r (NH) = 1.017(7) A; ∠ CCC = 125.7(2)°, ∠ CCN = 118.0(7), ∠ CNH = 110.3(8), ∠ CCNH = 58.5(9)°. Several excited states of the CC torsion were also searched for and are assigned.

Use of the anomalous Stark effect for the approximate determination of torsional energy splittings

Abstract An advantageous use of the anomalous Stark effect for the determination of the torsional energy splittings is proposed. The method is applicable in those cases where the dipole moment component connecting the torsional states of different parity has an experimentally observable magnitude. The analysis of the Stark spectrum observed in the spectrum of the N-cis lone-electron-pair gauche isomer of allylamine is presented as an example. The Stark effects of those transitions ( a -type and b -type) which are inside the same vibrational state (0 + and 0 − ), are used for the fitting of the μ a , μ b , and μ c dipole moment components as well as the torsional energy splitting Δ ν tors . The values obtained are compared to more accurate values derived by other methods. The possible use for predicting large torsional splittings is discussed.

Allycyanide gauche revisited

Abstract The microwave spectrum of the ground state of the gauche rotamer of allylcyanide (CH 2 =CHCH 2 CN) has been remeasured. The obtained rotational constants A = 19 707.9 ± 0.1, B = 2 619.74 ± 0.05 and C = 2 497.43 ± 0.05 (in MHz) were in good agreement with a structural model. The dipole moment components were also fitted as | μ a | = 3.50 ± 0.05, | μ b | = 1.70 ± 0.02 and | μ c | = 0.19 ± 0.04 (in Debye). The results are in both cases in good agreement with a CCCC dihedral angle near the expected 120°.

Microwave spectrum of ethylamine-gauche

Abstract The gauche rotamer of the ethylamine molecule was successfully assigned making ample use of MW-MW-DR-techniques. The transitions of the gauche form appear as quartets due to two large amplitude motions: the CN torsion and inversion. The rotational as well as interaction constants for all four states were fitted as well as their relative energies.