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The infrared and Raman data on the ring-puckering vibration in cyclobutane and cyclobutane-d8 have been reexamined including the coordinate dependence of the reduced mass in the Hamiltonian. This was done for the purpose of estimating the importance of these small terms in the determination of barrier heights for four-membered rings and also on the determination of the dihedral angle corresponding to the potential minimum. The conclusions reached are that there is an isotopic dependence of the barriers to planarity in cyclobutane and cyclobutane-d8 yielding a difference of ∼14 cm−1, but the precise value of the difference in barrier heights is ill determined. The higher-order kinetic energy terms in the Hamiltonian can account for a spread of ∼3 cm−1 in each of the barriers derived for cyclobutane and cyclobutane-d8, depending on the details of the model used for the vibration, but not a difference of 14 cm−1, which undoubtedly indicates the effects of coupling with other vibrational modes. It is also found that the derived values of the dihedral angles are quite sensitive to the details of the vibrational model, in fact, much more so than to the uncertainties in the bond distances and bond angles. A relationship between the potential constants derived for cyclobutane and cyclobutane-d8 assuming an effective constant reduced mass and those derived for a semirigid model is demonstrated.

The microwave spectrum of 1,2-dimethylenecyclobutane has been studied in the range 26.5–40 GHz using a Hewlett-Packard 8400C Stark-modulated spectrometer. The rigid rotor constants have been derived for the ground state (in MHz: A = 4925.22, B = 4089.88, C = 2301.67) and four excited states of the ring-puckering vibration. That the ring skeleton is planar is indicated by the smooth variation of the rotational constants with vibrational state and by the value of 1/2(Ia + Ib − Ic) which is consistent with only 4 hydrogen atoms out of the plane of the remaining atoms. Analysis of the Stark effect yields a dipole moment lying along the b-axis; μb = 0.457 ± 0.002D. A physically reasonable set of structural parameters which reproduce the ground state rotational constants has been derived by adjustment of the carbon skeleton parameters by a diagnostic least-squares procedure.

The microwave spectrum of thietanone-3 has been studied in the R -band range (26.5–40.0 GHz) using a Hewlett-Packard Model 8400C phase stabilized Stark-modulated microwave spectrometer. The rotational constants in the ground state (in MHz: A = 10205.06 ± 0.59, B = 3266.63 ± 0.01, C = 2559.70 ± 0.01) and ten excited states of the ring-puckering vibration have been derived from fitting a -type low- J R -branch transitions. The rotational constants have been used in the determination of a single minimum anharmonic potential function for the ring-puckering vibration of the form V = a ( Z 4 + BZ 2 ) with a = 9.81, B = 6.41 where Z is a reduced ring-puckering coordinate. From the analysis of the Stark effect, the dipole moment has been determined in the ground vibrational state ( μ a = 0.999 ± 0.003D) and 5 excited states of the ring-puckering mode. This variation of the dipole moment is reproduced by the above potential function and the expansion μ v = 0.982 + 0.074〈 Z 2 〉 v , where 〈 Z 2 〉 v is the expectation value of Z 2 in the v th vibrational state. A model calculation to reproduce the variation of the rotational constants has been performed. There is some indication that the CCC (sp 2 ) and CSC ring angles decrease slightly as the ring puckers but the puckering is accomplished primarily by decreasing the CCS angles. A reasonable set of structural parameters which reproduce the ground state rotational constants has been derived by a diagnostic least-squares procedure.

Experiments to detemine the signs of the imaginary damping factors in the resonance denominators of the expressions for the third order susceptibilities governing coherent anti‐Stokes and coherent Stokes Raman spectroscopy (CARS and CSRS) have been carried out using the ν1 line of β‐carotene. The experimental results are compared to the two different theoretical results predicted by the fully quantum mechanical and the semiclassical calculations for the third order susceptibilities for CARS and CSRS.

The microwave spectrum of thiane, a heterocyclic analog of cyclohexane, has been studied in the region 26.5–40 GHz. The molecule is a highly asymmetric rotor (κ = 0.050154). From the analysis of both the a -type and c -type transitions, the rotational constants determined are (in MHz): A = 3992.719, B = 3005.812, and C = 1914.683. A study of the Stark effect has yielded the dipole moment components (in Debye units) μ a = 1.684 ± 0.009, μ c = 0.578 ± 0.002, which give a total dipole moment of μ = 1.781 ± 0.010. Comparison of the spectral data from tetrahydropyran, thiane, and 1,4-thioxane demonstrates the similarity in structure of these three compounds. It is found that a very reasonable set of structural parameters can be found which adequately fits the spectral data of all three molecules.

The microwave spectrum of 2-cycloheptene-1-one, an unsaturated cyclic ketone, has been studied in the regions 26.5–40 and 7.0–12.4 GHz. An analysis of the ground-state “ a ”-type transitions yielded the rotational constants (in MHz): A = 2997.27, B = 2049.24, C = 1399.76. The “ a ”-type transitions of an excited vibrational state were also assigned, giving A = 3000.51, B = 2046.65, C = 1398.88. The centrifugal distortion constants, D J and D JK , were needed to fit the data adequately. A study of the Stark effect yielded the dipole moment components (in debye) μ a = 3.63 ± 0.023 and μ c = 0.882 ± 0.040. The μ b component could not be determined from the Stark effect data. These data are used to discuss the molecular conformation of cycloheptene-1-one.

The microwave spectrum of 1,4-thioxane, a heterocyclic analogue of cyclohexane, has been studied in the region 26.5–40 GHz. The molecule is a highly asymmetric rotor ( k = −0.0740917). From the analysis of the a -type transitions, the rotational constants determined are (in MHz) A = 4333.85, B = 3076.14 and C = 1991.96. A study of the Stark effect has yielded the dipole moment components (in Debye units) μ a = 0.290 ± 0.003, gm c = 0.0537 ± 0.0004, which give a total dipole moment of μ = 0.295 ± 0.003. These data are consistent with a chair conformation for the ring.

Under certain conditions, when the laser pump frequency approaches an electronic transition frequency, negative peaks have been observed in coherent anti‐Stokes Raman spectra. Two conflicting explanations of this phenomenon have been offered. We have noted that these two explanations lead to a different dependence of the intensity of the negative peaks on the power in the pump beam. We have experimentally determined this power dependence in the case of a 5×10−3M solution of N, N‐diethyl‐p‐nitrosoaniline in benzene and found that the intensity of the negative peaks varies with the square of the power in the pump beam. This result is consistent with the explanation that the negative peaks arise from a cross term between the background susceptibility and the imaginary part of the Raman susceptibility and not from the inverse Raman effect.

The low‐frequency vapor phase Raman spectra have been recorded for bicyclo[3.1.0]hexane, 3‐oxabicyclo[3.1.0]hexane, 6‐oxabicyclo[3.1.0]hexane, and 3,6‐dioxabicyclo[3.1.0]hexane. From the six observed ring‐puckering transitions for bicyclo[3.1.0]hexane, the ring‐puckering potential energy function was determined to be V(cm−1)=24.70(Z4+25.72Z2+ 9.28Z3), where Z is the reduced ring‐puckering coordinate. For each of the oxabicyclohexanes three transitions were observed for the puckering mode and these were in agreement with previously reported far‐infrared spectra. No puckering transitions were observed for the dioxabicyclohexane. For each of the four molecules, frequencies corresponding to twisting of the five‐membered ring and rocking of the three‐membered ring were also observed.

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.

The microwave spectrum of 7‐oxabicyclo[4.1.0]hept‐3‐ene has been studied and the rotational constants for the ground state are (in MHz) A=4073.15±0.01, B=3394.54±0.01, C=2174.07±0.01. Stark effect measurements have allowed determination of the dipole moment components μa=0.775±0.002 D and μc=1.770±0.005 D for a total dipole moment μ=1.932±0.005 D. Analysis of these data indicates the molecule has a boat conformation.

The excitation profile of the 1382 cm −1 vibration of the highly fluorescent molecule tetracene has been studied by means of coherent anti-Stokes Raman spectroscopy (CARS). By considering the behavior of the real and imaginary parts of the third-order susceptibility, calculated using the Albrecht theory modified for CARS, a means of calculating excitation profiles which scale like those obtained experimentally has been developed. The absolute value of the dimensionless vibronic displacement parameter was determined by comparing the experimental excitation profile with excitation profiles calculated for several different values of the displacement parameter and the results were used to calculate the internal coordinate change, between the ground and first excited electronic states, for the 1382 cm −1 mode.

The microwave spectrum of 2,3‐dihydro‐p‐dioxin(1,4‐dioxene) has been studied in the R‐band range (26.5–40.0 GHz) using a Hewlett‐Packard model 8400 Stark modulated spectrometer. The rotational constants for the ground state and four vibrationally excited states have been derived from fitting b‐type, low J R ‐branch transitions. The rotational constants for the ground state are (in megahertz) A = 5697.703±0.010; B = 4968.410±0.008; C = 2865.181±0.010. The presence of a C 2 symmetry axis has been established by analysis of the Stark effect yielding a single nonzero component of the dipole moment μ b = 0.939±0.008 D and by the effect of nuclear spin statistical weights on the relative intensities of the rotational transitions. That the symmetry point group is C 2 rather than C 2v is established by the value of (1/2)(Ia + Ib − Ic ) which is clearly too large for the C 2v structure. Model calculations to reproduce the rotational constants show that they are particularly sensitive to two structural parameters: the C=C–O angle (123.4±0.5°) and the twist angle, i.e., the angle between the C=C bond and C–C bond (29.9±1.5°). Consideration of the ring angles for a structure derived by diagnostic least squares shows little, if any, ring strain.

The microwave spectrum of 3,6‐dihydro‐2H‐pyran has been studied in the R‐band frequency range (26.5–40.0 GHz) with a Hewlett‐Packard Model 8400 Stark‐modulated microwave spectrometer. The spectrum is characterized by rather strong perpendicular transitions of a near oblate symmetric top. Transitions obeying a, b, and c dipole selection rules have been assigned and 24 low‐J R ‐branch transitions have been used to derive the rigid rotor constants for the ground vibrational state: (in MHz) A = 5085.152±0.012; B = 4849.365±0.013; C = 2712.487±0.013. Stark displacement measurements were made for several transitions and the dipole moment components determined by least squares fitting the displacements; (in Debye) |μa|=0.892±0.006; |μb|=0.507±0.010; |μc|=0.771±0.002 yielding a total dipole moment μtot = 1.283±0.005. Model calculations to reproduce the rotational constants and the observed dipole moment components indicate that the data are consistent with a twisted ring skeleton with the oxygen atom on one side ...

The excitation profiles for the ν1 and ν2 vibrations of trans‐ β‐carotene have been studied by means of coherent anti‐Stokes Raman spectroscopy (CARS). The experimental profiles are compared to those calculated using the Albrecht theory modified for CARS. In order to obtain meaningful results it was necessary to drastically lower the laser power, specifically the power of ω1, the beam in resonance with the electronic absorption. If the laser power was too high, the intensity of the β‐carotene lines did not scale with the square of the power in the pump beam due to saturation effects and, in fact, if the power was high enough the CARS signal was lost in the background emission. Due to the necessity of using low powers, photomultiplier detection was employed. The methods developed made it possible to detect β‐carotene in benzene solution at concentrations of ∼5×10−7M, which is considerably lower in concentration than previously observed by CARS.

The 42 fundamental frequencies of methylenecyclopentane, methylenecyclopentane-2,2,5,5-d4 and methylene-d2-cyclopentane-2,2,5,5-d4 are assigned from the vapor and liquid phase infrared spectra and the Raman spectra of liquid samples. Assignments are made on the basis of group frequencies, intensities, isotopic shifts, gas phase band contours, Raman depolarization ratios and comparison with similar molecules. The lowest frequency ring mode is observed as a combination band progression in the mid-infrared spectrum of methylenecyclopentane yielding a progression of bands beginning at 158.0 cm−1 and decreasing in frequency. These bands are interpreted in terms of a one-dimensional harmonic vibration with a small negative quartic anharmonicity. This interpretation is favored over that of pseudorotation in the methylenecyclopentane molecule.

A two‐dimensional Schrodinger equation appropriate to pseudorotation in five‐membered rings with the potential function V=(z1 2 + z2 2)2 + B (z1 2 + z2 2), where z1, z2 are reduced coordinates of ring bending and twisting, has been solved numerically by several different approaches. The result of the two‐dimensional approach, as opposed to the method often used of solving an angular equation alone, is to show a negative curvature in the pseudorotation energy level separations and an increase in the ``pseudorotational constant,'' β, in the excited states of the radial mode. These effects are a direct result of the finite central barrier for B < 0, given by B2/4. This increase in the pseudorotational constant in the excited state of the radial mode and the negative curvature of the transition frequencies had already been observed experimentally in the case of tetrahydrofuran and 1,3‐dioxolane.

The low frequency Raman spectrum of gaseous 2‐cyclohexene‐1‐one has been obtained at the full vapor pressure at ∼110°C using the 488 nm line of an argon ion laser at ∼2W power. 2‐cyclohexene‐1‐one has been found to exist with all of the heavy atoms except C5 essentially in a plane. The series of Q‐branch transitions in the Raman spectrum between 140–250 cm−1 have been assigned as Δv=2 transitions of a one‐dimensional double‐minimum oscillator. The vibarational mode is the motion of the CH2 group at the 5 position through the plane of the heavy atoms. The potential function in a reduced coordinate Z [V (cm−1) =27.49(Z4−11.67 Z2)] yields a barrier to planarity of 935±70 cm−1 (2.67±0.2 kcal/mole, 11.2±0.8 kJ/mole).

The microwave spectrum of 3,6‐dihydro‐2H‐pyran has been studied in the R‐band frequency range (26.5–40.0 GHz) with a Hewlett‐Packard Model 8400 Stark‐modulated microwave spectrometer. The spectrum is characterized by rather strong perpendicular transitions of a near oblate symmetric top. Transitions obeying a, b, and c dipole selection rules have been assigned and 24 low‐J R ‐branch transitions have been used to derive the rigid rotor constants for the ground vibrational state: (in MHz) A = 5085.152±0.012; B = 4849.365±0.013; C = 2712.487±0.013. Stark displacement measurements were made for several transitions and the dipole moment components determined by least squares fitting the displacements; (in Debye) |μa|=0.892±0.006; |μb|=0.507±0.010; |μc|=0.771±0.002 yielding a total dipole moment μtot = 1.283±0.005. Model calculations to reproduce the rotational constants and the observed dipole moment components indicate that the data are consistent with a twisted ring skeleton with the oxygen atom on one side ...

The microwave spectrum of 2,3‐dihydro‐p‐dioxin(1,4‐dioxene) has been studied in the R‐band range (26.5–40.0 GHz) using a Hewlett‐Packard model 8400 Stark modulated spectrometer. The rotational constants for the ground state and four vibrationally excited states have been derived from fitting b‐type, low J R ‐branch transitions. The rotational constants for the ground state are (in megahertz) A = 5697.703±0.010; B = 4968.410±0.008; C = 2865.181±0.010. The presence of a C 2 symmetry axis has been established by analysis of the Stark effect yielding a single nonzero component of the dipole moment μ b = 0.939±0.008 D and by the effect of nuclear spin statistical weights on the relative intensities of the rotational transitions. That the symmetry point group is C 2 rather than C 2v is established by the value of (1/2)(Ia + Ib − Ic ) which is clearly too large for the C 2v structure. Model calculations to reproduce the rotational constants show that they are particularly sensitive to two structural parameters: the C=C–O angle (123.4±0.5°) and the twist angle, i.e., the angle between the C=C bond and C–C bond (29.9±1.5°). Consideration of the ring angles for a structure derived by diagnostic least squares shows little, if any, ring strain.

The low‐frequency vapor phase Raman spectra have been recorded for bicyclo[3.1.0]hexane, 3‐oxabicyclo[3.1.0]hexane, 6‐oxabicyclo[3.1.0]hexane, and 3,6‐dioxabicyclo[3.1.0]hexane. From the six observed ring‐puckering transitions for bicyclo[3.1.0]hexane, the ring‐puckering potential energy function was determined to be V(cm−1)=24.70(Z4+25.72Z2+ 9.28Z3), where Z is the reduced ring‐puckering coordinate. For each of the oxabicyclohexanes three transitions were observed for the puckering mode and these were in agreement with previously reported far‐infrared spectra. No puckering transitions were observed for the dioxabicyclohexane. For each of the four molecules, frequencies corresponding to twisting of the five‐membered ring and rocking of the three‐membered ring were also observed.

The microwave spectrum of 1,3‐cycloheptadiene has been studied. The ground state and a number of excited vibrational states have been assigned and the rotational constants evaluated. A study of the Stark effect has yielded the dipole moment components for the ground state and two of the excited vibrational states. The total dipole moment for the ground state is 0.740 D. These data have been used to determine the conformation of 1,3‐cycloheptadiene. The carbon skeleton is planar except for the C6 carbon atom (Cs form).