A. Peter Cox

Microwave spectrum, structure, dipole moment and quadrupole coupling constants of cis and trans nitrous acids

Microwave spectra for eight isotopic species of cis and trans nitrous acids are reported. The planarity of both isomers has been accurately established on the basis of the inertial defects. Accurate structures, electric dipole moments and quadrupole coupling constants at nitrogen have been determined for both isomers. The structure of cis nitrous acid is found to be: O—H = 0.982, N—O(H)= 1.392, N—O = 1.185 A, ∠ NOH = 104.0 and ∠ ONO = 113.6°, and for trans nitrous acid: O—H = 0.958, N—O(H)= 1.432, N—O = 1.170 A, ∠ NOH = 102.1 and ∠ ONO = 110.7°. Stark effect measurements yield the following values for the components of the dipole moment for cis nitrous acid: µa= 0.306, µb= 1.389 and the total dipole moment µ= 1.423±0.005 D. Previous investigations of the Stark effect in the trans isomer have been refined to give µa= 1.378, µb= 1.242 and µ= 1.855±0.016 D. Orientation of the dipole moment has been established for both isomers from isotopic measurements, despite a moderately large vibrational change in µa for trans DNO2. Quadrupole coupling constants in the principal inertial axis system have been determined for cis nitrous acid to be: χaa= 2.05, χbb=–5.83 and χcc= 3.78 MHz, and for trans nitrous acid: χaa= 1.73, χbb=–5.28 and χcc= 3.55 MHz. The structural parameters are markedly different in the two isomers and have been discussed, together with the torsional barrier and relative stability of the isomers, in terms of a cis interaction. The structures and charge properties of the nitrous acids are compared with a range of related compounds.

Microwave l-type resonance transitions of the v6 = 1 state in CHF3 and CDF3: Accidental degeneracy and molecular structure

Abstract Observations of the direct l -type resonance transitions in the microwave spectrum of the v 6 = 1 state of CHF 3 and CDF 3 have been extended to J = 37. Accidental degeneracies were found between J = 34, K = 3 and 0 in both molecules enabling the C rotational constants of CHF 3 and CDF 3 to be determined, using 118 observed frequencies for CHF 3 and 104 for CDF 3 . After suitable correction the B and C rotational constants were used to determine the r 0 , r 2 , and r e structures for CHF 3 and CDF 3 . The equilibrium structure was determined to be CH = 1.091 ± 0.014 A , CF = 1.3284 ± 0.0031 A and ∠FCF = 108.58 ± 0.34°.

Microwave spectrum and structure of nitromethane

The microwave spectra of nitromethane and five isotopic species have been investigated. Ground-state moments of inertia for the m= 0 internal rotation state have been used to derive the following structure: C—N = 1.489 A, N—O = 1.224 A, ∠ONO = 125.3° and ∠NCH = 107.2°, Nuclear spin weightings for various |K–m| states, derived using non-rigid group theory, have been confirmed by relative intensity measurements in the microwave spectra. The same theory has been used to explain the unusually large increase (0.060 a.m.u. A2) in the inertial defect of CD3NO2.The structure and bonding of nitromethane is compared with some related molecules. The structure of the nitro group in nitromethane is shown to fit a series of NO2 compounds, between NO+2 and NO–2, where the parameters vary according to the electronegativity of the attached grouping.

The microwave spectrum of thioformaldehyde, CD2S, and CH2S: Average structure, dipole moments, and 33S quadrupole coupling

Abstract Microwave transitions up to J = 53 in the ground vibrational state of deuterothioformaldehyde, CD 2 S, were studied between 8 and 40 GHz. A detailed centrifugal distortion analysis yields accurate constants for comparison with force field values. The isotopic species 13 CH 2 S, CH 2 34 S, CH 2 33 S, 13 CD 2 S, CD 2 34 S, and CD 2 33 S were studied in natural abundance. Accurate average zero-point structures were determined for both CD 2 S and CH 2 S: CH 2 S CS =1.6138(4) CH = 1.0962(6) A ∠ HCH =116° 16(6)′, CD 2 S CS =1.6136(4) CD = 1.0931(4) A ∠ DCD =116° 25(5)′ Changes in the zero-point geometry for deuterium substitution were established. Quadrupole fine structure arising from the 33 S nucleus has been measured in CH 2 33 S and CD 2 33 S. Analysis gives the following coupling constants (for both molecules) as χ aa = −11.7 and χ bb - χ cc = 88.1 MHz. The dipole moment of CD 2 S was measured to be 1.6588(8)D and an accurate comparison with CH 2 S was made; the ratio of dipole moments CD 2 S CH 2 S was found to be 1.0062(4). The spectroscopic and bonding properties of CH 2 S will be compared with formaldehyde and other molecules.

Microwave spectrum, structure, barrier and dipole moment of trifluoronitrosomethane

The microwave spectra of three isotopic species of CF3NO have been measured giving an accurate structure with a significantly long CN bond length; in addition, the barrier to internal rotation of the CF3 top (V3 = 770 cal mol−1) and electric dipole moment (μ = 0.18 D) have been determined.

Microwave spectrum, structure and dipole moment of phenylacetylene

The microwave spectra of phenylacetylene, C6H5CCH, the deuterated species, C6H5CCD, and carbon–13 species at positions C(2), C(3), C(4), C(7) and C(8) have been measured and analysed. Inertial defects confirm the expected planarity of the molecule, and pronounced nuclear spin effects in the ground-state and vibrationally excited spectra confirm the C2v symmetry. The detailed structure is determined to be: C(1)C(2)= 1.388, C(2)C(3)= 1.396, C(3)C(4)= 1.398, C(1)C(7)= 1.448, C(7)C(8)= 1.208, and C(8)H = 1.055 A; C(6)C(1)c(2)= 120.8°, C(1)C(2)C(3)= 119.8°, C(2)C(3)C(4)= 119.9° and C(3)C(4)C(5)= 119.9°. The dipole moment has been accurately determined using new Stark-effect technique to be µ=µa= 0.656 ± 0.005 D and is found to be smaller for the deuterated species by 0.010 ± 0.006 D.The effect of the acetylenic group on the structure of the benzene ring is quite small, as expected from results of microwave studies for other substituted benzenes, while the acetylenic parameters are close to those of methylacetylene. The dipole moment is also near to that of methylacetylene (and other alkyl acetylenes) and is taken to indicate that it arises essentially from a π-system polarisation of the acetylenic group as suggested by recent molecular orbital calculations.

Microwave l-type resonance transitions of the v4 = 1 state in PF3: Detailed interactions and molecular structure

Abstract Observation of the direct l -type resonance transitions in the microwave spectrum of the v 4 = 1 state of PF 3 has been extended to J = 36. The w -type interaction, (Δ l = 0, Δ K = 6), has been found from measurements on the “forbidden” Stark trasitions in the K = 3 series. Also in this series a close accidental degeneracy was found between J = 30, K = 3 and 0, leading to new zero-field “forbidden” transitions through the r -type interaction (Δ l = 2, Δ K = −1) and to the determination of the C rotational constant. Nine spectroscopic parameters were determined using 140 observed frequencies including two “forbidden” trasitions. After suitable correction the B and C constants were used to determine the r 0 , r z , and r e structures for PF 3 . The equilibrium structure is estimated to be P-F = 1.561 ± 0.001 A and ∠FPF = 97.7 ± 0.2°.

Microwave spectra of CF3Br and CF3I. Structures and dipole moments

The microwave spectra of the 13C-species of CF3Br and CF3I have been measured with the aid of radiofrequency pumping of the quadrupole sub-levels. In addition, rotational data for the main species have been refined. The following zero-point average structures (rz) have been determined: CF3Br: C—F = 1.3265(23), C—Br = 1.9234(31)A, ∠FCF = 108.81(25)°, CF3I: C—F = 1.3285(23), C—I = 2.1438(27)A, ∠FCF = 108.42(23)°.Particular attention has been paid to the effect of isotopic variation in bond length in the structural calculations. The structures agree well with those recently obtained by combining electron diffraction and microwave data.Accurate Stark measurements have been performed for a number of transitions including the J= 3 → 4, K= 3, F= 7/2 → 9/2 transition of CF379Br and the J= 4 → 5, K= 3, F= 3/2 → 5/2 transition of CF3I. The results have been fitted using a diagonalization treatment of the quadrupole-Stark matrix, set up in the uncoupled basis. The dipole moment of CF3Br was determined to be 0.639(2) D and that of CF3I, 1.048(3) D. The structural properties of CF3Br and CF3I are shown to fit some trends with related molecules.

Centrifugal distortion in the microwave spectra of cis- and trans-nitrous acids. Determination of the quadratic potential functions and average structures

Centrifugal distortion shifts in the microwave spectra of cis and trans nitrous acids and their deuterated species have been analyzed using the first-order treatments of Hill and Edwards, and Watson. The two methods are in close agreement for the ground vibrational state of planar molecules. Centrifugal distortion constants thus derived have been used, together with the infra-red fundamentals and their isotopic shifts, to determine the quadratic potential function for each isomer. Eleven out of a possible sixteen force constants have been determined as follows (in mdyn A–1): for cis nitrous acid, ƒ11(OH)= 6.55, ƒ22(NO)= 11.18, ƒ33(N—O)= 2.73, ƒ44(NOH)= 0.88, ƒ55(ONO)= 2.46, ƒ23= 1.80, ƒ24= 0.22, ƒ34= 0.53, ƒ35= 0.35, ƒ45= 0.18 and ƒ66(torsion)= 0.167; and for trans nitrous acid, ƒ11= 7.18, ƒ22= 12.28, ƒ33= 2.22, ƒ44= 0.76, ƒ55= 2.45, ƒ23= 1.67, ƒ24= 0.12, ƒ34= 0.04, ƒ35= 0.41, ƒ45= 0.14 and ƒ66= 0.131.Using this potential function, average moments of inertia have been evaluated for cis and trans nitrous acid and their isotopic species allowing structures of the average configurations to be determined: for cis nitrous acid, O—H = 0.989, N—O(H)= 1.399, NO = 1.186 A, ∠ NOH = 103.9 and ∠ ONO = 113.6°, and for trans nitrous acid, O—H = 0.959, N—O(H)= 1.442, NO = 1.169 A, ∠ NOH = 102.1 and ∠ ONO = 110.6°. The near-zero inertial defects for the average moments of inertia show both isomers to be accurately planar in their average configurations. An equilibrium structure has been estimated for trans nitrous acid on the basis of a model cubic potential function.Differences in the harmonic force fields of the two isomers substantiate the marked difference in their structural parameters and have been taken as evidence for attraction between hydrogen and cis oxygen. The force fields and structures of the nitrous acids are compared with those of a range of related compounds.

Combined microwave-optical barrier determination for molecules with a heavy symmetric internal top: CF3NO and CF3CHO

Abstract The torsional data for CF 3 NO have been rein vest igated. A model with a single degree of freedom and three adjustable parameters is sufficient to fit data to v = 8 in the electronic ground state. For CF 3 NO we obtain F o = 1.9822(42) cm −1 , V 3 = 238.4(1.6) cm −1 and V 6 = −5.8(1.6) cm −1 or F o = 1.9894(66) cm −1 , F 3 = −0.194(55) cm −1 and V 3 = 239.3(1.9) cm −1 . A similar treatment for CF 3 CHO gives F o = 1.97(14) cm −1 , V 3 = 305(25) cm −1 and V 6 = −8.7(1.2) cm −1 . A need for a re-examination of the torsional fundamental is indicated for CF 3 CHO. These studies support the general conclusion that for a heavy internal top the internal rotation constant, F o , required to fit a range of torsional splittings is different from that calculated from structural considerations alone. The difference indicates a large change in F with torsional averaging.