R. N. Nandi

Microwave spectrum, dipole moment, and conformation of ethanedithiol

The microwave spectrum of 1,2‐ethanedithiol has been investigated. Measurements yield rotational constants having values A=9292.451±0.006, B=2239.194±0.002, C=1935.557±0.002 MHz; and dipole moment components μa=0.683±0.011, μb=1.749±0.044, μc=0.774±0.034 D. Analysis of these data shows that the molecule adopts an unsymmetrical non‐hydrogen‐bonded conformation.

Microwave spectral study of cyclopentyl acetylene

Abstract The microwave spectrum of cyclopentyl acetylene has been observed and analyzed in the 26.5–40 GHz frequency range. Rotational constants have been obtained for the axial and equatorial conformers as follows (in MHz): A = 4264.820 ± 0.429, B = 2168.762 ± 0.009, C = 2032.080 ± 0.009 for the axial; A = 6349.374 ± 0.399, B = 1765.183 ± 0.012, C = 1480.549 ± 0.013 for the equatorial. Relative intensity measurements showed the equatorial conformer to be more stable by 270 ± 70 cal/mole. A low-frequency ring mode having a fundamental frequency of 40 ± 20 cm −1 was observed up to v = 6. The conformational parameters and pseudorotational potential energy surfaces for various monosubstituted cyclopentanes have been compared and discussed.

Microwave spectral studies of rotational isomerism in substituted ethanethiols: 2-Aminoethanethiol and 2-chloroethanethiol

Abstract Microwave spectra were observed and analyzed for 2-aminoethanethiol and 2-chloroethanethiol. The amino compound exists in two gauche rotameric conformations, one exhibiting an intramolecular SH⋯N hydrogen bond. The hydrogen-bonded conformer lies higher in energy by 274 ± 90 cal mole −1 and has the following rotational constants (in MHz): A = 12 040.1 ± 11.3, B = 3352.24 ± 0.03, and C = 2881.99 ± 0.03. For the non-hydrogen-bonded conformer the rotational constants (in MHz) are A = 11 929.9 ± 10.2, B = 3395.01 ± 0.03, and C = 2877.82 ± 0.03. Dipole moment measurements for the H-bond conformer led to μ a = 2.68 D, μ b = 0.88 D, and μ c = 0.37 D, while for the non-H-bond form the values are μ a = 1.51 D, μ b = 0.0 D, and μ c = 0.62 D. In the case of chloroethanethiol, the only assigned spectral lines were the unresolved J → J + 1 a -type bands of a trans conformation. For this molecule the combination rotational constant B + C has the value 2955.17 ± 0.02 MHz for the 35 Cl species and 2879.73 ± 0.02 MHz for the 37 Cl species.

Microwave spectrum, structure and dipole moment of 2,3-diazabicyclo[2.2.2]-octene

Abstract Investigation of the microwave spectrum of diazabicyclooctene (C 6 H 10 N 2 ) has led to determination of the ground vibrational state rotational constants as follows: A = 2677.580 ± 0.002, B = 2642.397 ± 0.002, C = 2519.603 ± 0.004 MHz. Measurements of the second-order Stark effect have also been performed. These show the molecule to have the expected C z axis of symmetry with μ c = μ T = 3.51 ± 0.06 Debye. The dipole moment and qualitative structural features of this accidentally near-spherical rotor are compared to those of related molecules.

Microwave spectral study of bicyclo[2.2.2]octene and bicyclo[2.2.2]octadiene

Microwave spectral studies of bicyclo[2.2.2]octene (C8H12) and bicyclo[2.2.2]octadiene (C8H10) have led to the following rotational constants in MHz: for C8H12, A=2576.721±0.002, B=2508.998±0.002, C=2462.272±0.009; and for C8H10, A=2730.116±0.002, B=2650.520±0.002, C=2631.710±0.002. The dipole moments of the C2v molecules have also been obtained, the values along the symmetry axis being μc=0.253±0.001 D for C8H12 and μc=0.432±0.002 D for C8H10.

Microwave-microwave double-resonance using a Fabry-Perot cavity spectrometer

The design and operation of a relatively simple, yet versatile spectrometer for microwave–microwave double resonance (MMDR) studies is described. The system consists of a high‐Q, semiconfocal Fabry–Perot cavity resonator which is crossed at right angles by a second free‐space microwave beam. We excite the cavity with a low‐power unmodulated signal source, while the free‐space irradiation source is square‐wave frequency modulated and is of moderately high power (several hundred milliwatts). Both microwave sources are scannable; the signal source is locked to the cavity while the pumping klystron is free running. The signal (cavity) frequency has been operated in the 15–40‐GHz range, and pump frequencies can be selected and easily implemented above or below the signal microwave frequency.