Microwave spectrum,dipole moment,and conformation of 3,6-dioxabicyclo[3.1.0.]hexane

The microwave spectrum of 3,6-dioxabicyclo[3.1.0.]hexane has been obtained. The rotational lines of one ring conformation only have been observed and assigned. Ground state rotational constants are A₀ = 6287.302 ± 0.011 MHa, B₀ = 4683.546 ± 0.008 MHz, and C₀ = 3358.517 ± 0.089 MHz. The diploe moment components obtained from Stark effect measurements are μ_a = 0.276 ± 0.010 D and μ_c = 2.47 ± 0.04 giving μ = 2.485 ±0.040 for the dipole moment of the molecule. The rotational constants and dipole moment components obtained experimentally can be satisfactorily explained only if the boat form is the most stable ring conformation.     

The microwave spectrum,dipole moment,and conformation of 3-oxabicyclo(3.1.0.)hexane

The microwave spectrum of 3-oxabicyclo(3.1.0.)hexane has been studied in the range 26.5–40 GHz (R-band) with a Hewlett Packard Model 8400 spectrometer. Both a and c-type R-branch transitions were used to derive the rotational constants for the ground state and first two excited states of the ring-puckering mode. The data are consistent with a single stable conformation, in agreement with a previous far-infrared study (1) and this is shown to be the boat conformation, as was the case with the similar molecules cyclopentene oxide (2, 3) (6-oxabicyclo(3.1.0.)hexane) and 3,6-dioxabicyclo(3.1.0.)hexane (1, 4). The rotational constants for the ground state are (in MHz) A = 6038.06; B = 4432.47; C = 3303.43 yielding κ = ? 0.174268. The electric dipole moment components of the ground state (in Debye units) are |μ_a| = 1.36 ± 0.02; |μ_c| = 1.03 ± 0.02 yielding a total dipole moment μ = 1.71 ± 0.03.     

Microwave spectrum,structure, and dipole moment of bicyclo[3.1.0]hex-2-ene

The microwave spectra of the normal and four monosubstituted ¹³C isotopic species of bicyclo[3.1.0]hex-2-ene have been observed and analyzed. For the normal species the rotational constants (in megahertz) are: Λ = 6306.121 ± 0.006, B = 4516.667 ± 0.004, C = 3208.823 ± 0.002. From the complete data set, a partial r_s heavy-atom structure has been obtained as well as a complete effective structure. The r_s distances are found to be C₁C₅ = 1.521 ± 0.001 Å, C₁C₂ = 1.494 ± 0.010 Å, C₅C₆ = 1.482 ± 0.006 Å, C₁C₆ = 1.522 ± 0.007 Å. The overall effective structure shows the five-membered ring to be only slightly nonplanar (by ca. 6°), and the three-membered ring to be rather sharply inclined with respect to the five-membered ring (dihedral angle C₁C₅C₆-C₁C₅C₄ = 113.5°). Dipole moment measurements for the symmetryless molecule yielded values of |μ_a| = 0.166 ± 0.009, |μ_b| = 0.209 ± 0.015, |μ_c| = 0.119 ± 0.001, |μ_T| = 0.292 ± 0.012 D.     

Microwave spectrum and conformation of thiomorpholine

The microwave spectrum of normal thiomorpholine (CH₂CH₂SCH₂CH₂NH) was investigated within the region 8–40 GHz, and that of N-deuterothiomorpholine (CH₂CH₂SCH₂CH₂ND) within the region 26.5–40 GHz. The observed spectra are due to the chair equatorial conformers. The rotational constants of both isotopic species were determined for the ground states and for two vibrationally excited states. The dipole moment components and quadrupole coupling constants of normal thiomorpholine and the iminohydrogen r_s coordinates were also determined.     

The photoelectron spectrum and molecular geometry of bicyclo[2.1.1]hexane

The He(I) photoelectron spectrum of bicyclo[2.1.1]hexane shows excellent correlation with a theoretical spectrum based on the eigenvalues obtained in an ab initio calculation at the theoretical molecular geometry. By contrast, a calculation performed at the experimental geometry exhibits an effect of enhanced ring strain in showing a first ionization energy which is considerably lower than the experimental value. This observation offers experimental support for the recent claim that the published structure for bicyclo [2.1.1]hexane based on electron-diffraction measurements is incorrect. It also accounts for inconsistencies observed in the calculated values of some spin—spin coupling constants for substituted bicyclo[2.1.1]hexanes.     

The microwave spectrum of 2,4-dioxabicyclo[3.1.0]hexan-3-one

The R band (26.5–40 GHz) microwave spectrum of 2,4-dioxabicyclo[3.1.0]hexan-3-one is reported. Rotational constants for the ground vibrational state of the common ¹²C₄¹H₄¹⁶O₃ and ¹³C₁, ¹³C₆ isotopically substituted species (the latter observed in natural abundance) have been evaluated. In addition rotational constants of the V_B = 1 to V_B = 5 quanta associated with the bending vibration of the five membered ring have been determined. A partial r_s structure has been calculated:

$\text{r(}\text{C}{}_1\text{?}\text{C}{}_5\text{) = 1.497± 0.016}\text{A}\text{?}\text{, r(}\text{C}{}_1\text{?}\text{C}{}_6\text{) = r(}\text{C}{}_6\text{?}\text{C}{}_5\text{) = 1.522 ± 0.015}\text{A}\text{?}$

,

$\text{∠}\text{C}{}_6\text{C}{}_1\text{C}{}_5\text{= ∠}\text{C}{}_1\text{C}{}_5\text{C}{}_6\text{= 60°32′ ± 1°36′, ∠}\text{C}{}_1\text{C}{}_6\text{C}{}_5\text{= 58°′ ± 1°47′}$

. With certain assumed molecular information a least squares fit yields the following parameters:

$\text{β = 68.5 ± 0.02°, r(}\text{C}{}_1\text{O}{}_2\text{= 1.408 ± 0.004}\text{A}\text{?}$

,

$\text{∠}\text{C}{}_5\text{C}{}_1\text{O}{}_2\text{= 105.8 ± 0.02°, ∠}\text{C}{}_1\text{O}{}_2\text{C}{}_3\text{= 108.10 ± 0.03°}$

,

$\text{∠}\text{O}{}_2\text{C}{}_3\text{O}{}_4\text{= 112.8 ± 0.02°}$

.     

Microwave spectrum and dipole moment of bicyclo[2.1.1]hexan-5-one

The microwave spectrum of bicyclo[2.1.1]hexan-5-one has been analyzed, resulting in a total dipole moment of 3.18 ± 0.02 D with an orientation of 1.7 ± 1.7° with respect to the carbonyl bond axis. This dipole moment is about 0.3 D larger than that of most other carbonyl compounds, but it is reasonably comparable to the even higher values which have been reported for bicyclo[2.1.1]hexan-2-one (3.35 ± 0.02 D) and for cyclopentanone (3.25 ± 0.02 D). The rotational constants for the ground vibrational state are A = 4448.347 ± 0.011, B = 2693.349 ± 0.008, and C = 2673.300 ± 0.006, the uncertainties being standard deviations determined from a least-squares fit.     

Microwave spectrum,conformation, and dipole moment of 4-thiacyclohexanone

The microwave spectra of 4-thiacyclohexanone in the ground state and eight vibrationally excited states have been studied in the frequency region 18.0–40.0 GHz and the corresponding rotational constants have been determined. The following values of the ground-state rotational constants (MHz) were obtained from the analysis of the a-type transitions: A = 3935.149 (0.031), B = 1829.444 (0.001), and C = 1364.609 (0.001). Analysis of the Stark effect gives for the dipole components (in Debye units) μ_a = 1.409 (0.002), μ_c = 0.391 (0.064). These data are consistent with a chair conformation for the ring. A phisically reasonable set of structural parameters which reproduce the ground-state rotational constants has been derived. A qualitative estimate of the low-frequency vibrational modes was obtained from relative-intensity measurements. The lowest vibrational frequency is believed to be a ring-bending mode and it occurs at 77 ± 22 cm^?1 while the ring-twisting mode is at 204 ± 27 cm^?1.     

Microwave spectrum,dipole moment,and conformation of 3-methylcyclopentanone

The rotational spectrum of 3-methylcyclopentanone has been observed in the frequency region from 18.0 to 26.5 GHz. Both a-type and b-type transitions in the ground vibrational state and a-type transitions in five excited states have been assigned. The ground state rotational constants are determined to be A = 5423.32 ± 0.18, B = 1949.51 ± 0.01, and C = 1529.59 ± 0.01 MHz. Analysis of the measured quadratic Stark effects gives the dipole moment components ∥μ_a∥ = 2.97 ± 0.02, ∥μ_b∥ = 1.00 ± 0.03, ∥μ_c∥ = 0.18 ± 0.06, and the total dipole moment ∥μ_t∥ = 3.14 ± 0.03 D. These data are consistent with a twisted-ring conformation with a methyl group in the equatorial position.     

Microwave spectrum and conformation of vinyl mercaptan: The syn rotamer

Rotational spectra of vinyl mercaptan (ethenethiol) CH₂CHSH and its isotopic modification CH₂CHSD have been studied by microwave spectroscpy. The molecule has been found to exist in two rotameric forms, syn and anti, associated with different orientations of the SH bond with respect to the vinyl framework. In this paper results are reported for the more stable syn form which is shown to be planar with ground state rotational constants A = 49 815.28(6) MHz, B = 5835.716(14) MHz, C = 5222.081(11) MHz, D_J = 2.85(17) kHz, D_JK = ?33.22(2.08) kHz, and δ_J = 0.425(65) kHz. Spectra have also been observed for the first and second excited states of the SH torsional vibration and the first excited state of the CCS angle bending mode. The dipole moment of the syn rotamer is μ_a = 0.813(1), μ_b = 0.376(4), and μ_total = 0.896(3) D.     

Microwave spectrum and conformation of vinyl mercaptan: The anti rotamer

Microwave spectra of the anti rotamer of vinyl mercaptan and its SD isotopic species have been studied in the frequency range 12–60 GHz. For the normal species rotational and centrifugal distortion constants have been obtained for the ground and first three excited states of the SH torsional mode, the ground state values being A = 49 422.75(5) MHz, B = 5 897.215(9) MHz, C = 5 279.436(9) MHz, D_J = 3.12(11) kHz, D_JK = ?38.50(1.71) kHz, and δ_J = 0.498(51) kHz. An approximate potential function for the SH torsion in the vicinity of the anti conformation, derived using the observed variation of rotational constants with vibrational quantum number, reveals the presence of a small potential barrier of 19 cm^?1 at the planar conformation. The v = 0 state lies above this barrier so the molecule is essentially planar in the ground state in spite of the observed negative value for the inertia defect (?0.1976(2) a.m.u.Ų). The anti rotamer is found to be 50 ± 25 cm^?1 less stable than the syn rotamer. The dipole moment has the ground state values μ_a = 0.425(10), μ_b = 1.033(10), and μ_total = 1.117(14) D and is shown to vary considerably with vibrational quantum number. Evidence for significant structural changes in going from the syn rotamer to the anti rotamer is also presented.     

Microwave spectrum,conformation, and dipole moment of divinyl ether

The microwave spectrum of divinyl ether has been observed and a, b, and c type rotational transitions of one conformer assigned. This conformer has rotational constants closely related to the cis-trans planar form. The inertial defect and dipole moment reveal that it is not planar. This nonplanarity almost certainly results because of strong repulsion between the β hydrogen of the cis vinyl group and the α hydrogen of the trans vinyl group. The c type transitions are split 53 MHz by inversion tunneling. The dipole moment has been obtained from Stark effect measurements and is 0.782 debye.     

Microwave spectrum,quadrupole coupling constant,and conformation of 1-cyanocyclohexene

The microwave spectrum of 1-cyanocyclohexene has been investigated in the frequency regions 8–12.4 and 18–26.5 GHz. A-Type transitions in the ground state and three excited states have been assigned. The rotational constants in the ground state were determined to be A = 4565.98 ± 0.46, B = 1423.44 ± 0.01, and C = 1136.17 ± 0.01 MHz. From the experimental data, it was suggested that the molecule has an equilibrium half-chair conformation similar to those of cyclohexene and 1-fluorocyclohexene. From the hyperfine splittings of the ¹⁴N nucleus, the quadrupole coupling constant, X_aa, was found to be 4.2 MHz.     

Microwave spectrum,conformation, and quadrupole coupling constants of cyclopentyl chloride

The microwave spectra of the normal and two isotopic species of cyclopentyl chloride have been observed and analyzed. For the normal isotopic species the rotational constants (in MHz) are A = 4547.77 ± 0.01, B = 2290.22 ± 0.01, and C = 2073.34 ± 0.01. From the rotational constant data, it has been shown that the stable molecular conformation is the bent axial form. Quadrupole coupling constants have been measured for the ³⁵Cl nucleus, the values being (in MHz) χ_aa = ?23.70 ± 0.10, χ_bb = 32.33 ± 0.36, and χ_cc = ?8.63 ± 0.37. When transformed to the CCl bond axis system, the coupling constants confirm the axial structure. Extensive vibrational satellite structure, presumably arising from the pseudorotational ring mode with a fundamental frequency of 52 ± 5 cm^?1, has been observed and assigned. No spectral evidence has been observed for a second stable molecular conformer.     

Microwave spectrum,structure, and dipole moment of 7-oxabicyclo[2.2.1]heptane

The microwave spectrum of 7-oxabicyclo[2.2.1]heptane has been assigned in the ground and two excited vibrational states. Relative intensity measurements indicate that these two vibrations have wavenumbers of 120(30) and 330(30) cm^?1. The dipole moment obtained from Stark effect measurements is 1.621(10) D. The molecule is shown to have C_2v symmetry and the assignment of the two singly substituted ¹³C species gives the following skeletal structure: C₁-C₂ = 1.537(5) Å; C₂-C₃ = 1.551(5) Å; C₁-O = 1.452(10) Å; ?C₁OCC₄ = 95.3(10)° φ = 113.1(5)°.     

Far-infrared spectra of ring compounds. XII. Far-infrared spectra,ring-puckering vibration,and molecular conformation of some analogs of bicyclo(3.1.0.)hexane

The far-infrared spectra of bicyclo(3.1.0.)hexane, 3-oxabicyclo(3.1.0.)hexane, and 3,6-dioxabicyclo(3.1.0.)hexane exhibit series of Q branches in the frequency range 250–125 cm^?1, like those observed for 6-oxabicyclo(3.1.0.)hexane (cyclopentene oxide) by Carreira and Lord (J. Chem. Phys.51, 2735 (1969)). The Q branches are interpreted as single quantum jumps of a one-dimensional ring-puckering vibration governed by a potential function of the form V(cm^?1) = A(Z⁴ + BZ² + CZ³), where Z is a reduced ring-puckering coordinate.The potential parameters for bicyclo(3.1.0.)hexane are A, 24.31 cm^?1; B, 26.67; C, 9.63; for 3-oxabicyclo(3.1.0.)hexane, 27.77 cm^?1, 20.63, 7.84; and for 3,6-dioxabicyclo(3.1.0.)hexane, 25.25 cm^?1, 17.23, 7.33. They were determined by an iterative least-squares fit to the observed four, seven, and nine Q branches, respectively. These potential functions all have only a single minimum, implying a single stable conformation. The dipole moment, μ_rms, for 3,6-dioxabicyclo(3.1.0.)hexane in benzene solution was determined to be 2.50 D, clearly indicating that the stable conformation is the boat form. The boat conformation had been determined to be the stable form for cyclopentene oxide from a microwave study by Lafferty (J. Mol. Spectrosc.36, 84 (1970)). There is no direct evidence for the preferred conformation of the remaining two molecules but consideration of torsional interactions about the 1–2 and 4–5 bonds as well as the similarity of the spectra and potential functions for these molecules suggests that the boat conformation is the stable form of all of these molecules.     

Microwave spectrum,conformation, quadrupole coupling constants,and barrier to internal rotation of methoxyamine

The microwave spectra of three isotopic species of methoxyamine (CH₃ONH₂) have been studied. For the normal species the ground-state rotational constants are A = 42488 ± 150 MHz, B = 10049.59 ± 0.03 MHz, and C = 8962.85 ± 0.03 MHz. From these data and those from the -NHD and -ND₂ species, the amino protons have been shown to occupy a symmetrical trans position relative to the methyl group. The barrier to internal rotation of the methyl group has been found to be 873 ± 15 cm^?1 by analysis of ground-state splittings. Analysis of hyperfine splittings has yielded the ¹⁴N quadrupole coupling constants, which have the following values for the normal isotopic species: χ_aa = 3.63 ± 0.03 MHz, χ_bb = ?3.69 ± 0.07 MHz, and χ_cc = 0.06 ± 0.07 MHz.     

Microwave spectrum of ketene

A new Stark-modulated submillimeter-wave spectrometer is described. This spectrometer has been used to analyze the microwave spectrum of three isotopomers (heavy atoms) of ketene. The rotational constants determined have been used to calculate the structure of ketene using a variety of methods. The question of planarity of ketene is also addressed. High-resolution microwave measurements have been used to determine the spin-rotation interaction in CH₂¹³CO.     

Microwave spectrum of DNO

The microwave spectrum of deuterated nitroxyl DNO has been observed and analyzed. The molecule was produced by the reaction of D with NO in a flow system. Both a-type and b-type transitions have been observed and the resulting rotational constants, A = 315450.3 ± 4.8 MHz, B = 38731.5 ± 1.5 MHz, and C = 34354.0 ± 1.5 MHz, are in good agreement with those of the lower electronic state ¹A′ for the electronic transition of DNO observed by Dalby. The quadrupole coupling constants for nitrogen are χ_aa = 1.03 ± 0.40 MHz, χ_bb = −6.13 ± 0.26 MHz, χ_cc = 5.10 ± 0.26 MHz. The components of the electric dipole moment of DNO have been determined to be μ_a = 1.18 ± 0.04 D and μ_b = 1.22 ± 0.04 D, giving a total dipole moment μ_total = 1.70 ± 0.05 D. The half lifetime of the molecule varies from 1 to 40 sec, depending on the condition of the surfaces of the absorption cell, which is much longer than the values reported previously.     

Microwave spectrum of tropone

The microwave spectrum of tropone (2,4,6-cycloheptatriene-1-one) has been obtained and assigned. The observation of a statistical weight intensity alternation indicates that the molecule has C_2v symmetry. Relative intensity measurements show the molecule to possess a low wavenumber vibration whose 1-0 interval is 60 ± 20 cm^?1. The dipole moment is found to be 4.1 ± 0.3 D by the “rate-of-growth” method.