Nancy S. True

Low resolution microwave spectroscopy, part IV1: The conformation of isopropyl formate, isopropyl fluoroformate, isopropyl chloroformate, isopropyl cyanoformate, and isopropyl trifluoroacetate

Abstract Low resolution microwave spectra of isopropyl formate, fluorofonnate, chloroformate, cyanoformate, and trifluoroacetate each display strong a-type band series associated with a single rotamer. Experimental B+C values for these compounds are: 4641±5; 3172±4; 2275±1/2226±1 MHz ( 35 Cl/ 37 Cl); 2257±1 MHz; and 1509±4 MHz, respectively. In each case the experimental values agree with a τ - 1 (Oue5fbCOC) = 0°, τ 2 (COCH) ~ 40° structure indicating a modest internal rotation barrier when the methine C—H bond is syn eclipsed with the carboxyl framework. A potential function for the internal rotation of an isopropyl group extrapolated from the known functions in methyl and ethyl formate and one calculated by ab initio methods support the model derived from the low resolution microwave spectra. The single isopropyl conformer observed can be viewed as a combination of the two most stable forms of ethyl esters.

Low resolution microwave spectroscopy: Part 14. Conformations of S-isopropyl thioesters and a high resolution study of S-isopropyl thiofluoroformate

Abstract A high resolution microwave assignment of S -isopropyl fluorothioformate yielded the following rotational constants: A 0 = 4196(1) MHz, B 0 = 1374.23(3) MHz and C 0 = 1159.80(4 MHz, compatible with a syn-gauche (τ 1 (OCSC) = 0°, τ 2 (CSCH) ∼ 40°) conformer. Low resolution microwave spectra of S -isopropyl trifluorothioacetate, cyanothioformate and chlorothioformate each display strong a -type band series compatible with either syn-syn (τ 1 (OCSC) = 0°, τ 2 (CSCH) = 0°) or syn-gauche (τ 1 (OCSC) = 0°, τ 2 (CSCH) ∼ 60°) conformers. Experimental B+C values are 1211.7(6), 1839.6(7) and 1814.6(5), (1774.4(5), 37 Cl) MHz for the trifluorothioacetate, cyanothioformate and chlorothioformate, respectively. S -isopropyl trifluorothioacetate also displays a weak band series with a B+C value of 1310.2(5) MHz compatible with a syn-anti (τ 1 (OCSC) = 0°, τ 2 (CSCH) = 180°) structure. Torsionally excited species freely rotating about τ 2 have been previously observed in S -ethyl thioesters but are not observed in S -isopropyl thioesters indicating a higher torsional barrier for the S -isopropyl compounds.

Direct observation of free rotor states in compounds with hindred internal rotation

Abstract Low-resolution microwave spectra of species torsionally excited above an internal rotation barrier are observed. Although the population of a single excited state is not large, the total population of these spectrally unresolved states is often sufficient to yield intense band spectra.

Methyl barrier to internal rotation and quadrupole coupling constants in S-methylchlorothioformate

Abstract Internal rotation A-E splittings have been observed in the ground state for both 35Cl and 37Cl isotopic species of S-methylchlorothioformate. The values V 3 ( 35 Cl ) = 893 ± 20 and V 3 ( 37 Cl) = 890 ± 20 cal/mole have been obtained. The anaalysis of the hyperfine structure gave χ aa ( 35 Cl ) = −49.2 , χ bb ( 35 Cl ) = 22.4 and χ aa ( 37 Cl ) = −39.0 , χ bb ( 37 Cl) = 18.3 MHz . Only the syn-conformation of the methyl group with respect to the carbonyl group has been observed. A partial r0 structure is given.

Microwave spectroscopic study of the conformations, relative energies, and dipole moments of ethyl cyanoformate

Abstract The microwave spectrum of ethyl cyanoformate displays a-type band spectra from three nearly prolate conformers. High-resolution spectra of the two more stable species have been assigned. One form, designated extended, has rotational constants A″ = 6453.3(4) MHz, B″ = 1500.47(6) MHz, C″ = 1236.36(6) MHz, which are consistent with a syn-anti [τ1 (Oue5fbCOC) = 0°, τ2 (COCC) = 180°] structure. The second form, labeled compact, has rotational constants A″ = 6787.8(7) MHz, B″ = 1549.38(8) MHz, C″ = 1406.80(8) MHz, which are consistent with a syn-gauche [τ1 (Oue5fbCOC) = 0°, τ2 (COCC) ∼ 90°] structure. The extended form is marginally more stable, ΔE = 55 ± 27 cm−1. The extended conformer has dipole moment components μa = 4.44(7), μb ∼ 0 D and the compact conformer has dipole moment components μa = 4.25(7), μb = 0, μc = 1.08(23) D. The third conformer (relative energy 600 ± 140 cm−1) has the most intense band series even at −63°C. the bands of this conformer are unresolvable into individual rotational transitions.

Microwave spectrum and rotational isomers of ethyl fluoroformate

Abstract The microwave spectrum of ethyl fluoroformate displays strong a-type R branch transitions from two rotameric forms. One species (extended form) has rotational constants A 0 = 9191.3(9) MHz, B 0 = 2112.61(1) MHz, C 0 = 1756.73(1) MHz which are consistent with a syn-anti (τ 1 (OCOC) = 0°, r 2 (cocc) = 180°) planar heavy atom structure. The second species (compact form) has rotational constants A 0 = 7760(3) MHz, B 0 = 2388.38(4) MHz, C 0 = 2102.47(3) MHz which are consistent with a syn-gauche (τ 1 (ococ) = 0°, τ 2 (cocc) ~ 90°) structure. The two conformational forms have approximately equal energy (0 ± 40 cm −1 ). Four vibrational satellites of the extended species have been analyzed yielding a torsional frequency around the O-ethyl bond of 70(10) cm −1 . Three vibrational satellites attributed to the O-ethyl torsion of the compact species have been analyzed yielding a vibrational frequency of 90(10) cm −1 . Approximate Fourier coefficients of a three term potential function for internal rotation about the O-ethyl bond have been determined. Vibrational satellites attributed to the first excited states of the O-ester torsion have been analyzed for both conformers. The torsional vibrational frequency around the O-ester bond is 110(15) cm −1 for the extended conformers and 120(20) cm −1 for the compact.

Low resolution microwave spectroscopy. II. The conformation of 2,2,2‐trifluoroethyl vinyl ether

The low resolution microwave spectrum of gaseous 2,2,2‐trifluoroethyl vinyl ether displays an intense series of a‐type bands and a weak b‐type series which identify the species to have an s‐cis‐planar configuration, τ (C=C–O–C) = 0° and τ (C–O–C–CF3) = 180° [Fig. 1(b)]. This confirms the determination by Charles, Cullen, and Owen based on infrared evidence and analogy to unsubstituted ethyl vinyl ether. No other species is observed which excludes any significant population of an s‐trans‐planar species, τ (C=C–O–C) = 180° and τ (C–O–C–CF3) = 180° [Fig. 1(d)]. Any other species significantly present in the vapor would have to be relatively nonpolar, i.e., have τ (C–O–C–CF3) < 90°.

Microwave spectrum and conformation of isopropyl fluoroformate

Abstract The microwave spectrum of isopropyl fluoroformate is characterized by intense a -type R -branch transitions from one conformational species. The rotational constants of the ground state, A 0 = 4967.0(8) MHz, B 0 = 1704.69(2) MHz, C 0 = 1468.86(1) MHz and κ = −0.8651(2) are consistent with a τ 1 (Oue5fbCOC) = 0°, τ 2 (COCH) ~35° structure. This structure can be viewed as a combination of the two conformational species found in ethyl fluoroformate. Two vibrational satellites having rotational constants A 0 = 4963(5) MHz, B 0 = 1694.11(7) MHz. C 0 = 1471.43(4) MHz and A 0 =4998(6) MHz, B 0 = 1705.21(7) MHz, C 0 = 1471.10(4) MHz have been assigned.