Search

The microwave spectrum of phthalan has been observed in the frequency range of 18–70 GHz. The spectra of the ground and ten excited vibrational states have been assigned and rotational and quartic centrifugal distortion constants have been obtained from semirigid rotor fits to the observed line frequencies. The microwave spectrum is not consistent with either the C2v equilibrium conformation deduced from the far infrared spectrum or the C2 equilibrium conformation obtained from the single vibronic level fluorescence spectra. The microwave data indicates that the bending of the five membered ring is a very anharmonic vibration with a double minimum potential function with a low barrier and that phthalan therefore has a Cs equilibrium conformation. The previous spectroscopic data can be reinterpreted in a manner consistent with this and gives a barrier height of 38 cm-1. The twisting of the five membered ring is a reasonably harmonic vibration and its fundamental wave-number is ∼ 180 cm-1. The microwave dat...

The conformation and ring-puckering vibration of 2-methoxyloxetane have been studied using microwave spectroscopy and ab initio computations. The microwave spectra of the ground and first four excited states of the ring-puckering vibration have been observed in the frequency range 8–40 GHz and the rotational and quadratic centrifugal distortion constants have been determined. Vibrational energy separations have been obtained from relative intensity measurements. The electric dipole moment (in D) of the ground vibrational state has been determined from Stark effect measurements as μ a = 0.0187 (5), μ b = 1.852 (2), μ c = 0.08 (3), and μ t = 1.854 (3). The vibrational energy separations and the vibrational dependence of the rotational constants suggest that the ring-puckering vibrational has an asymmetric single minimum potential function and also gave a revised assignment of the far infrared spectrum (J. Mol. Struct. 56 (1979) 157). A combined fit to the rotational constants, vibrational energy separations and far infrared vibrational frequencies has been used to determine a reduced potential for the ring puckering vibration. The partial derivatives of the rotational constants with respect to the reduced ring-puckering coordinate show that the equilibrium conformation has the methyl group in the equatorial position. Ab initio computations of the ring-puckering potential function have been made using 6-31G * orbitals and full geometry optimization. These computations also show the molecule to have an asymmetric single minimum potential function with an equatorial equilibrium conformation.

Microwave measurements on the ground and first eight excited states of the ring-puckering vibration of butadiene sulfone have been extended to millimeter wavelengths. The microwave spectra of the same vibrational states of α,α′-D 4 butadiene sulfone have been observed. For both isotopomers the Coriolis interaction between the v = 0 and v = 1 states has been analyzed to give the energy separation between these two states. These data and the variation of the rotational constants have been used to derive reduced potential functions for the ring-puckering vibration. The barrier to ring inversion is 49(2) cm −1 for butadiene sulfone and 44(2) cm −1 for the α,α′-D 4 isotopomer. The ring-puckering vibrational dependence of the quartic centrifugal distortion constants, including a small dependence of Δ J and δ J , has been accounted for.

The microwave spectrum of 3-methyloxetane has been observed in the frequency range 12.4–40.0 GHz in order to investigate the conformational behaviour of this molecule. The spectra of the ground vibrational state, the first five excited states of the ring-puckering vibration and four other excited vibrational states have been assigned and their rotational and quartic centrifugal distortion constants have been obtained. The ring-puckering vibrational energy separations obtained from microwave relative intensity measurements do not appear to be consistent with those predicted from a ring-puckering potential function obtained from far infrared spectroscopy. A revised assignment of the far infrared spectrum gives a potential function consistent with the microwave relative intensity measurements. This potential function is preferred because of its superior ability to account for the ring-puckering vibrational dependence of the rotational and centrifugal distortion constants. Although this is a slightly asymmetric double minimum potential function, the top of the barrier lies below the lowest vibrational level and therefore in its ground vibrational state 3-methyloxetane effectively has a planar ring. The four excited states which do not belong to the ring-puckering progression based on the ground vibrational state are assigned to a ring-puckering progression based on the first excited state of the methyl torsion. The first two members of this progression show relatively large changes in their rotational constants relative to the ground vibrational state indicating a strong interaction between the ring-puckering and methyl torsion vibrations. The electric dipole moment has been determined from Stark effect measurements as μa = 1.910 (5) D, μc= 0.77 (2) D and μ = 2.06 (1) D for the ground vibrational state. The μa component and the total electric dipole moment decrease with excitation of the ring-puckering vibration.

The ring puckering in butadiene sulphone has been investigated using microwave spectroscopy and ab initio computations. Microwave spectra of the ground and first eight excited states of the ring‐puckering vibration have been observed. A two state analysis of the vibration–rotation interaction for the v=0 and v=1 states gives an energy separation between these states of 4.97 (3) cm−1. This separation and the vibrational dependence of the rotational constants have been used to derive the reduced potential function V(X)=4.7(X4−6.5X2) for the ring‐puckering vibration. This potential function gives a barrier to ring inversion of 50 (11) cm−1 and an equilibrium dihedral angle of ≊20°. Ab initio computations using STO 3‐21G* orbitals and full geometry optimization give a barrier height of 88 cm−1 and an equilibrium dihedral angle of 19.2°. The ab initio computations predict structural relaxation including a rocking of the sulphone group during the ring‐puckering motion. This is supported by calculations of the v...

An attempt has been made to account for the ring-puckering vibrational dependence of the quartic centrifugal distortion constants of 3-methylthietan. The calculations indicate revised assignments for the excited vibrational states in the microwave spectrum and for some of the transitions in the far-infrared spectrum of the ring-puckering vibration. The resulting potential function has a slightly higher barrier to ring inversion for the more stable equatorial conformer and a larger energy difference between the equatorial and axial conformers than that proposed previously.

The microwave spectrum of 4-methylenecyclohexene has been observed in the frequency range 12–40 GHz and the spectra of the ground and nine excited vibrational states of a single conformer have been assigned. Relative intensity measurements give 104(27) cm −1 and 161(33) cm −1 for the two lowest vibrational frequencies. The microwave spectrum is consistent with a high barrier to ring inversion.

The microwave spectrum of γ-butyrolactone has been reinvestigated in order to determine small vibrational splittings associated with the ring inversion. For the ground vibrational state the inversion splitting has been measured directly as 30.3 MHz by the observation of the µc rotation–inversion transitions. The inversion splitting in the first excited state of the two ring puckering vibrations has been determined through analysis of a type Coriolis interactions. In the first excited state of the 148 cm–1 vibration the inversion splitting is 609 MHz while for the 225 cm–1 it is 2930 MHz. These inversion splittings indicate that the 148 cm–1 vibration is associated with a single minimum potential function and the 225 cm–1 vibration with a double minimum potential function but that there is some interaction between the two vibrations. A one-dimensional reduced potential function has been derived for the 225 cm–1 vibration and gives a barrier to ring inversion of 8.0 kJ mol–1(670 cm–1).

The microwave spectrum of 4-methylenecyclohexene has been observed in the frequency range 12–40 GHz and the spectra of the ground and nine excited vibrational states of a single conformer have been assigned. Relative intensity measurements give 104(27) cm −1 and 161(33) cm −1 for the two lowest vibrational frequencies. The microwave spectrum is consistent with a high barrier to ring inversion.

The microwave spectrum of 5,6-dihydro-2H-thiopyran has been observed between 12.4 and 40 GHz. Rotational and quartic centrifugal distortion constants have been obtained for the ground and seven excited vibrational states. The fundamental wavenumbers of the two lowest vibrations have been obtained from relative intensity measurements as ν36= 134 (12) cm–1 and ν35= 219 (18) cm–1. The electric dipole moment of the ground vibrational state has been determined from Stark effect measurements as [in D, 1 D (Debye)≈ 3.335 64 × 10–30 C m], µa= 1.35 (1), µb⩽ 0.1, µc= 0.74 (2), with a total dipole moment of µ= 1.54 (2). The rotational constants and relatively large µc component of the electric dipole moment are consistent with 5,6-dihydro-2H-thiopyran having a twisted or half-chair conformation. A molecular structure has been derived from the ground vibrational state rotational constants and planar moments of inertia using the method of predicate observations. The equilibrium twist angle of the ring is determined as τ= 32.5 (1.5)°.

The rotational spectrum of α-angelicalactone has been analyzed in the frequency range 18.0–40.0 GHz. The internal rotation barrier of the methyl group has been determined in the ground and four vibrationally excited states from the A-E splittings. The results indicate a possible rotational between the methyl torsion and the ring puckering mode. The ground state rotational parameters are consistent with a planar ring skeleton. The dipole moment components obtained from Stark displacements are μa = 3.16(1) D and μb = 2.59(2) D with a total value of 4.08(2) D.

The microwave spectrum of 2,3-dihydrofuran has been reinvestigated and measurements for the ground and first five excited states of the ring puckering vibration have been extended to higher frequencies and rotational quantum numbers in order to study the vibrational dependence of the rotational and centrifugal distortion constants. The ring puckering potential function derived by Green from the far infrared spectrum does not reproduce the vibrational dependence of the rotational constants well. A slightly different potential function is derived which gives a reasonable fit both to the far infrared spectrum and the rotational constants. This changes the barrier to ring inversion from 1.00 kJ mol −1 to 1.12 kJ mol −1 . The vibrational dependence of the centrifugal distortion constants is accounted for satisfactorily by the theory developed by Creswell and Mills. An attempt to reproduce the vibrational dependence of the rotational and centrifugal distortion constants using the ring puckering potential function and a simple model for this vibration has very limited success.

A re-investigation of the microwave spectra of 2,5- and 2,3-di-hydrothiophene was performed. For the 2,5 isomer lines up to J = 40 were measured for the ground and first five vibrationally excited states of the ring-puckering mode. The centrifugal distortion constants in the ground and excited states show the expected behaviour for a single mininum potential function. For 2,3-dihydrothiophene evidence was found for a Coriolis interaction between the ground vibrational state and the first excited state of the ring-puckering vibration. A preliminary analysis of this effect places the vibrational energy separation ΔE 01 at 348 ± 4 MHz.

A reinvestigation of the microwave spectrum of N-methyl-p-fluoroaniline shows this molecule to have low lying excited vibrational states associated with the inversion and torsion of the HNCH3 group similar to N-methylaniline. The experimental data indicate a greater tendency towards a planar configuration at the nitrogen atom and a lower barrier to inversion compared to p-fluoroaniline.

Microwave spectra of the normal and amine-deuterated isotopic species of N -methyl-aniline in the ground and some low-lying excited vibrational states have been observed. The inertial defect indicates that the dihedral angle of the N-CH 3 , bond with respect to the ring plane is somewhat less than that for the N—H bonds in aniline. The position of the amino-hydrogen atom is very poorly determined by the isotopic substitution method because of large zero-point effects. The excited vibrational states are consistent with a double-minimum potential for the inversion of the HNCH 3 group and there is some evidence for a lower barrier than in aniline. The excited states of the HNCH 3 torsion indicate a barrier in the range 8 2 −1 to the internal rotation of this group. No splitting of the ground-state lines attributable to the torsion of the methyl group has been observed, which implies a barrier of V 3 > 8 kJ mol −1 .

The microwave spectrum of 1,3,5-trithiane has been observed at a temperature of ca. 360 K in the frequency range 26–70 GHz. Only the spectrum of the symmetric top chair conformer is identified. Spectra of the ground vibrational state and some excited states involving the two lowest degenerate vibrations are assigned.

The microwave spectrum of 3-cyclohexen-1-one has been observed in the frequency range 18–40 GHz and transitions for the ground and 19 excited vibrational states have been assigned. Relative intensity measurements give the frequency of the two lowest vibrations as 74 and 147 cm−1. The molecule has a nonplanar ring and the molecular structure obtained from a molecular mechanics computation gives rotational constants in reasonable agreement with those obtained from the spectra.

The microwave spectrum of 1,3,5-trithiane has been observed at a temperature of ca. 360 K in the frequency range 26–70 GHz. Only the spectrum of the symmetric top chair conformer is identified. Spectra of the ground vibrational state and some excited states involving the two lowest degenerate vibrations are assigned.

A reinvestigation of the microwave spectrum of N-methyl-p-fluoroaniline shows this molecule to have low lying excited vibrational states associated with the inversion and torsion of the HNCH3 group similar to N-methylaniline. The experimental data indicate a greater tendency towards a planar configuration at the nitrogen atom and a lower barrier to inversion compared to p-fluoroaniline.

The microwave spectrum of 3-cyclohexen-1-one has been observed in the frequency range 18–40 GHz and transitions for the ground and 19 excited vibrational states have been assigned. Relative intensity measurements give the frequency of the two lowest vibrations as 74 and 147 cm−1. The molecule has a nonplanar ring and the molecular structure obtained from a molecular mechanics computation gives rotational constants in reasonable agreement with those obtained from the spectra.

The microwave spectrum of the normal isotopic species of p-fluoroanisole has been measured for the ground vibrational state and for the first excited torsional state about the Ar—O bond. The rigid rotor rotational constants were found to be: v= 0, A= 4883.87 MHz, B= 972.475 MHz, C= 815.540 MHz; v= 1, A= 4868.62 MHz, B= 972.675 MHz, C= 816.488 MHz. The values of Δ(Ia+Ib–Ic) for the two states (Δ°= 3.476 a.m.u. A2, Δ1= 4.413 a.m.u. A2) were found to be consistent with a planar framework of heavy atoms.

The microwave spectra of the ground and five excited states of the ring puckering vibration of 2,5-dihydrothiophen S,S-Dioxide show this vibration to have a double minimum potential function with a barrier of ca. 100 cm–1 to planarity of the ring.

The microwave spectra of the ground and first three excited states of the inversion vibration of 2-aminopyrimidine-NH2 and for the ground and first excited inversion state of 2-aminopyrimidine-ND2 have been assigned. Vibration rotation interactions prevent an accurate determination of the amine group geometry but there is considerable evidence to indicate a greater tendency towards molecular planarity in 2-aminopyrimidine than in a number of other amine molecules. SCF computations using STO/3G orbitals support this suggestion.

The microwave spectra of three isotopic species (C6H511B35Cl2, C6H511B35Cl37Cl and C6H510B35Cl2) of phenylboron dichloride have been observed. The spectra of the first four excited torsional states of C6H511B35Cl2 and the first two excited torsional states of C6H511B35Cl37Cl have been assigned, the variation of the inertial defect with torsional quantum number gives a torsional frequency of 37 ± 2 cm–1 and a barrier to internal rotation of V2= 13.6 ± 0.8 kJ mol –1. The rs structure of the BCl2 group is rB–Cl= 176.0 ± 0.8 pm and ClBCl = 121.7 ± 0.5°. With the assumption of regular benzene ring (rC–C= 139.7 pm and rC–H= 108.4 pm) a least squares fitting procedure to the observed rotational constants gives the following structural parameters rB–Cl= 176.0 ± 0.8 pm, rB–C= 155.6 ± 1.4 pm and ClCl = 121.8 ± 0.4°.

Microwave spectra of the ground and first three excited torsional states of C6H511BF2 and for the ground and first two excited torsional states of C6H510BF2 have been assigned. The variation of the inertial defect with torsional quantum number shows the molecule to be planar. The torsional frequency has been determined as ν= 49 cm–1 and the barrier to internal rotation as Y2= 1114 cm–1(13.3 kJ mol–1). With the assumption of a regular benzene ring (rC—C= 1.397 A and rC—H= 1.084 A) the geometry of the rest of the molecule is calculated to be rC—B= 1.551 A, rB—F= 1.330 A and FBF = 116.0°.

Microwave spectra of the ground and first excited vibrational states of the normal and amine deuterated species of 3-aminopyridine and 4-aminopyridine have been assigned and the structure of the amine group has been determined in these two molecules. The structure of the amine group in the aminopyridines has been derived from ab initio molecular orbital calculations using single determinant molecular orbital wave functions constructed from standard minimal basis sets of contracted Gaussian orbitals. The experimental and theoretical results are found to be in broad agreement with each other.

Microwave spectra have been observed for the normal and acid deuterated isotopic species of the s-cis and s-trans rotamers of acrylic acid in the ground and a number of excited torsional states. Both rotameric forms are planar with the hydroxyl group adopting the cis conformation relative to the carbonyl group. The dipole moments determined from Stark effect measurements are [graphic omitted].The difference in zero point vibrational energy between the two rotamers and the 1 â†� 0 torsional frequencies have been determined from relative intensity measurements as E°s-trans—E°s-cis= 58 ± 20 cm–1, νs-cis= 105 ± 20 cm–1, νs-trans= 95 ± 20 cm–1. The potential function hindering the internal rotation is essentially two-fold with a barrier of 1340 ± 500 cm–1(16.0 ± 6.0 kJ mol–1) at an angle of 89 ± 4° from the s-cis configuration.

The microwave spectrum of the normal isotopic species of p-fluoroanisole has been measured for the ground vibrational state and for the first excited torsional state about the Ar—O bond. The rigid rotor rotational constants were found to be: v= 0, A= 4883.87 MHz, B= 972.475 MHz, C= 815.540 MHz; v= 1, A= 4868.62 MHz, B= 972.675 MHz, C= 816.488 MHz. The values of Δ(Ia+Ib–Ic) for the two states (Δ°= 3.476 a.m.u. A2, Δ1= 4.413 a.m.u. A2) were found to be consistent with a planar framework of heavy atoms.