Microwave spectrum, structure, tautomeric, and conformational composition of 4-vinylimidazole

The microwave spectra of the two conformers each, of the 1H and 3H tautomers of 4-vinylimidazole, have been measured in the 48-72 GHz spectral region. The 4-vinylimidazole was generated in situ by the facile decarboxylation of urocanic acid at its vaporization temperature of 220 °C. The recognition of this reaction casts doubt on the reliability of a previous published spectroscopic study apparently mistakenly thought to be of uncontaminated vaporized urocanic acid, a natural product of great interest in skin cancer etiology. Quantum chemical theoretical predictions of the structures of each of ten possible conformers∕tautomers of urocanic acid and four of 4-vinylimidazole were performed at the ab initio MP2∕cc-pVTZ level, with vibrational predictions at the B3LYP∕cc-pVTZ and M062X∕cc-pVTZ levels. The predicted values of rotational constants for all the urocanic acid species were found to be quite inconsistent with those of the four observed spectra. For the 4-vinylimidazole isomers, the calculated relative energies suggested that all four species would have substantial equilibrium mole fractions at 220 °C. The isomers were identified by matching the observed and calculated rotational constants. The resulting assignment was found to be consistent with the predicted and observed (14)N nuclear quadrupole hyperfine multiplet patterns for a suitable rotational transition, and with the observed versus empirically calculated inertial defects. With one exception, the predicted structures were found to be planar. Resembling the case of 1-vinylimidazole, where one conformer is nonplanar, one isomer of 4-vinylimidazole was found to be quasiplanar. This seems to belong to a class of spontaneous symmetry-breaking observed in the molecular structure of some otherwise planar vinyl aromatic compounds.     

Microwave spectrum and conformational composition of 2-chloroethylisocyanide

2-Chloroethylisocyanide (ClCH(2)CH(2)N≡C) has been synthesized, and its microwave spectrum has been investigated in the 20-97 GHz spectral region. The spectra of (35)Cl and (37)Cl isotopologues of two conformers have been assigned. The Cl-C-C-N chain of atoms is antiperiplanar in one of these rotamers and synclinal in the second. The energy difference between the two forms has been obtained from relative intensity measurements. It was found that the antiperiplanar conformer is favored over the synclinal form by 4.3(8) kJ/mol. Quantum chemical calculations at the CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory have been performed. Most, but not all, of the spectroscopic constants predicted in these calculations are in good agreement with their experimental counterparts. The theoretical calculations correctly predict that the 2-chloroethylisocyanide exists as a mixture of an antiperiplanar and a synclinal conformer, with the former about 3.5 kJ/mol more stable than the latter. Both methods of calculations find that the antiperiplanar rotamer has a symmetry plane. The dihedral angle formed by the Cl-C-C-N link of atoms of the synclinal form is 67° according to the CCSD calculations. It is estimated from a comparison with the experimental rotational constants that this dihedral angle is uncertain by ±3°.     

Microwave spectrum and conformational composition of 2-fluoroethylisocyanide

The microwave spectrum of 2-fluoroethylisocyanide, FCH(2)CH(2)N≡C, has been investigated in the whole 50-120 GHz spectral region. Selected portions of the spectrum in the range of 18-50 GHz have also been recorded. The microwave spectra of the ground state and vibrationally excited states of two conformers have been assigned. Accurate spectroscopic constants have been derived from a large number of microwave transitions. The F-C-C-N chain of atoms is antiperiplanar in one of these rotamers and synclinal in the second conformer. The energy difference between the two forms was obtained from relative intensity measurements. It was found that the synclinal conformer is favored over the antiperiplanar form by 0.7(5) kJ/mol. Quantum chemical calculations at the high CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory were performed. Most, but not all, of the spectroscopic constants predicted in these calculations are in good agreement with the experimental counterparts. The theoretical calculations correctly indicate that the F-C-C-N dihedral angle in the synclinal form is about 67° but underestimate the magnitude of the gauche effect and erroneously predict the antiperiplanar rotamer to be 1.3-1.6 kJ/mol more stable than the synclinal conformer.     

Microwave spectrum and conformational composition of 3-fluoropropionitrile (FCH2CH2CN)

The microwave spectrum of 3-fluoropropionitrile, FCH(2)CH(2)C≡N, has been investigated in the whole 17-75 GHz spectral region. Selected portions of the spectrum in the 75-95 GHz have also been recorded. The microwave spectra of the ground state as well as of three vibrationally excited states of each of two conformers have been assigned. The spectra of the vibrationally excited states belong to the lowest torsional and bending vibrations. The F-C-C-C chain of atoms is exactly antiperiplanar in one of these rotamers and synclinal in the second conformer. The F-C-C-C dihedral angle is 65(2)° in the synclinal form. The energy difference between the two forms has been obtained from relative intensity measurements performed on microwave transitions. It was found that the antiperiplanar conformer is more stable than the synclinal form by 1.4(5) kJ/mol. It is argued that the gauche effect is a significant force in this compound. Quantum chemical calculations at the high CCSD(full)/cc-pVTZ, MP2(full)/cc-pVTZ, and B3LYP/cc-pVTZ levels of theory have been performed. Most, but not all, of the theoretical predictions are in good agreement with experiment.     

Microwave spectrum,conformational preference,barrier to internal rotation,and centrifugal distortion of 1-amino-2-propanol

The microwave spectrum of CH₃CH(OH)CH₂NH₂ has been investigated in the 26.5–39.7 GHz region. One rotamer with an intramolecular hydrogen bond formed between hydroxyl and ammo groups was assigned. This conformation is also characterized by having the methyl group anti to the amino group. Other forms, if they exist, must be at least 1 kcal mole^?1 less stable. Four vibrationally excited states belonging to three different normal modes were assigned and the barrier to internal rotation of the methyl group was found to be 3173 ± 100 cal mole^?1.     

Microwave spectrum, conformational composition, and intramolecular hydrogen bonding of (2-chloroethyl)amine (ClCH2CH2NH2)

The microwave spectrum of (2-chloroethyl)amine, ClCH(2)CH(2)NH(2), has been investigated in the 22-120 GHz region. Five rotameric forms are possible for this compound. In two of these conformers, denoted I and II, the Cl-C-C-N chain of atoms is antiperiplanar, with different orientations of the amino group. The link of the said atoms is synclinal in the three remaining forms, III-V, which differ with respect to the orientation of the amino group. The microwave spectra of four of these conformers, I-IV, have been assigned. In two of these rotamers, III and IV, the amino group is oriented in such a manner that rare and weak five-membered N-H···Cl intramolecular hydrogen bonds are formed. The geometries of conformers I and II preclude a stabilization by this interaction. The energy differences between the conformers were obtained from relative intensity measurements of spectral lines. The hydrogen-bonded conformer IV represents the global energy minimum. This rotamer is 0.3(7) kJ/mol more stable than the other hydrogen-bonded conformer III, 4.1(11) kJ/mol more stable than II, and 5.5(15) kJ/mol more stable than I. The spectroscopic work has been augmented by quantum chemical calculations at the CCSD/cc-pVTZ and MP2/6-311++G(3df,3pd) levels of theory. The CCSD rotational constants and energy differences are in good agreement with their experimental counterparts.     

Reactions and polymerization of 1-trityl-4-vinylimidazole

1-Trityl-4-vinylimidazole was prepared by direct tritylation of 4(5)-vinylimidazole and polymerized using a free radical initiator. Poly(1-trityl-4-vinylimidazole) was hydrolyzed using aqueous acetic acid to give poly[4(5)-vinylimidazole]. The poly[4(5)-vinylimidazole], which was obtained from the hydrolysis of poly(1-trityl-4-vinylimidazole), was compared with poly[4(5)-vinylimidazole] prepared directly from 4(5)-vinylimidazole for differences in stereochemistry. The stereochemistry of both polymers was found to be similar by high-resolution NMR. Thus, the trityl does not influence the stereochemistry of poly[4(5)-vinylimidazole]. The reaction of 1-trityl-4-vinylimidazole with n-butyllithium gave 2-lithio-1-trityl-4-vinylimidazole. This intermediate was used to prepare 2-substituted 4(5)-vinylimidazoles, which are new monomers that can be polymerized using free radical initiators.     

Rotational spectrum and conformational composition of cyanoacetaldehyde, a compound of potential prebiotic and astrochemical interest

The rotational spectrum of cyanoacetaldehyde (NCCH(2)CHO) has been investigated in the 19.5-80.5 and 150-500 GHz spectral regions. It is found that cyanoacetaldehyde is strongly preferred over its tautomer cyanovinylalcohol (NCCH═CHOH) in the gas phase. The spectra of two rotameric forms of cyanoacetaldehyde produced by rotation about the central C-C bond have been assigned. The C-C-C-O dihedral angle has an unusual value of 151(3)° from the synperiplanar (0°) position in one of the conformers denoted I, while this dihedral angle is exactly synperiplanar in the second rotamer called II, which therefore has C(s) symmetry. Conformer I is found to be preferred over II by 2.9(8) kJ/mol from relative intensity measurements. A double minimum potential for rotation about the central C-C bond with a small barrier maximum at the exact antiperiplanar (180°) position leads to Coriolis perturbations in the rotational spectrum of conformer I. Selected transitions of I were fitted to a Hamiltonian allowing for this sort of interaction, and the separation between the two lowest vibrational states was determined to be 58794(14) MHz [1.96112(5) cm(-1)]. Attempts to include additional transitions in the fits using this Hamiltonian failed, and it is concluded that it lacks interaction terms to account satisfactorily for all the observed transitions. The situation was different for II. More than 2000 transitions were assigned and fitted to the usual Watson Hamiltonian, which allowed very accurate values to be determined not only for the rotational constants, but for many centrifugal distortion constants as well. Two vibrationally excited states were also assigned for this form. Theoretical calculations were performed at the B3LYP, MP2, and CCSD levels of theory using large basis sets to augment the experimental work. The predictions of these calculations turned out to be in good agreement with most experimental results.     

Palladium(II) Complexes of 1-vinylimidazole

Two new co-ordination compounds of Pd^II with 1-vinylimidazole of the formulae [PdL₄]Cl₂·3H₂O and trans-[PdL₂Cl₂], where L is a 1-vinylimidazole molecule, have been obtained. The compounds were characterised by spectroscopic, molar conductivity, thermogravimetric and magnetochemical measurements. Single crystal X-ray structure analyses of the complexes were also carried out. The compounds are diamagnetic with square-planar coordinatination around the palladium(II) ions. Other physico-chemical properties of the both complexes are compatible with their structures.     

Microwave spectrum of ethynyl-cyclohexane

The ground state microwave rotational spectra of the axial and equatorial chair forms of ethynyl-cyclohexane are investigated in the frequency region 8–26.5 GHz. The spectrum of the first vibrationally excited state of the e-conformer is also assigned and intensity measurements locate this state at 130 ± 30 cm^?1. Using the ground state rotational constants obtained for the two isomers, a least-squares structural analysis is performed.     

Microwave spectrum of n-methylvinylamine

The microwave spectrum of a new transient molecule has been observed by pyrolysis of 2-methylaziridine. The rotational constants determined are A = 17413.011(14), B = 5974.342(4), and C = 4586.645(4) MHz. The analysis of the spectrum leads to the conclusion that the origin of the spectrum is cis-N-methylvinylamine. The molecular constants have been estimated by ab initio MO calculation, and they are consistent with the experimental results.     

Microwave spectrum of cyclohexanone

The rotational spectrum of cyclohexanone has been observed within the frequency region from 18.0 to 40.0 GHz. Transitions in the ground state and six excited states have been assigned. The ground state rotational constants are (in MHz) A = 4195.316 +- 0.059, B = 2502.627 ± 0.005 and C = 1754.443 ± 0.005.From information obtained from relative intensity measurements, these excited states are estimated to be ~ 100 cm^?1 above the ground state for the first excited state of the ring-bending mode and ~ 180 cm^?1 for the first excited state of the ring-twisting mode.     

Microwave spectrum of ethylamine-gauche

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.     

Microwave spectrum of sulfolane

The microwave spectrum of sulfolane has been investigated in the frequency region 26.5 to 40.0 GHz. The rotational constants have been derived for the ground state (in MHz: A = 3780.814 ± 0.326, B = 2047.243 ± 0.006 and C = 1899.572 ± 0.006) and three excited states of the ring puckering vibration. Possible interpretations of the effect of nuclear spin statistical weights on the relative intensity measurements and of the analysis of the variation of the rotational constants with the vibrational quantum number are discussed. Two situations may be compatible with the available data: pseudorotation with the bent conformer lowest in energy or a near planar structure with a small barrier at the planar configuration with respect to the bending mode.     

Microwave spectrum of cyclohexanecarbonitrile

The microwave rotational spectrum of cyclohexanecarbonitrile was investigated in the frequency region 8–40 GHz. From the measured transition frequencies the rotational constants of the two molecular were derived (equatorial isomer: A = 4238.77, B = 1399.172, C = 1128.845 MHz; axial isomer: A = 3005.58, B = 1763.483, C = 1558.615 MHz). Assuming the values of 1.531, 1.096 and 1.159 Å, respectively, for the CC, CH and CN distances, and supposing that the ring structure has the same symmetry as in cyclohexane, the following structural parameters were also obtained: equatorial isomer CCC (carbon ring) = 111.40°, r(CCN) = 1.489 Å, HCCN = 107.42°; axial isomer CCC (carbon ring) = 111.65°, r(CCN) = 1.489 Å, HCCN = 105.53°.     

Microwave spectrum and structure of isoxazole

The investigation of the microwave spectrum of isoxazole ($\text{ONCHCHC}$H) has been continued. The assignment of transitions up to J = 42 gave accurate rotational and distortion constants. The hyperfine splitting of the J = 0 → 1 transitions could be partially resolved and values for the quadrupole coupling constants were obtained.The use of microwave-microwave double resonance modulation in place of Stark effect modulation allowed the assignment of six vibrationally excited states and of the monosubstituted isotopic species containing ¹³C, ¹⁵N and ¹⁸O in their natural abundances. Kraitchman's equations for planar molecules were used to derive the r_s-structure of the isoxazole ring. Application of a least squares technique for singular systems of normal equations gave structure parameters which optimally reproduce the observed changes of all three moments of inertia under isotopic substitution.