F. Hegelund

Molecular equilibrium structures from experimental rotational constants and calculated vibration–rotation interaction constants

rotation interaction constants a r . The vibration‐rotation interaction constants have been calculated for 18 single-configuration dominated molecules containing hydrogen and first-row atoms at various standard levels of ab initio theory. Comparisons with the experimental data and tests for the internal consistency of the calculations show that the equilibrium structures generated using Hartree‐Fock vibration‐rotation interaction constants have an accuracy similar to that obtained by a direct minimization of the CCSD~T! energy. The most accurate vibration‐rotation interaction constants are those calculated at the CCSD~T!/cc-pVQZ level. The equilibrium bond distances determined from these interaction constants have relative errors of 0.02%‐0.06%, surpassing the accuracy obtainable either by purely experimental techniques ~except for the smallest systems such as diatomics! or by ab initio methods.

The harmonic force field and ground-state average structure of allene

Abstract Existing spectroscopic data for allene-H 4 , -D 4 , and -1,1-D 2 are improved and augmented by gas-phase Raman and solid-phase infrared studies. A number of 13 C vibration frequencies are identified in natural abundance by each technique. A total of 58 input data enable 22 of the 23 force field parameters to be determined, the 13 C frequency shift data removing the ambiguity of choice between two sets of A 1 and B 2 species force constants. The remaining interaction force constant is constrained to the predicted ab initio value of Botschwina and Pulay. The force field is used to determine the ground-state average ( r z ) structure of allene. Observed trends in the H-D isotopic effects on the r z structures of ethylene, allene, and ketene are in accordance with those expected.

High-resolution infrared rovibrational studies of the A1 species fundamentals of isotopic ketenes

The four A1-species fundamentals of ketene have been analyzed at a resolution of 0.05 cm−1. Assignments are made for J ≤ 36 and Ka ≤ 7, and asymmetry splittings observed for Ka ≤ 2. Although individual subband analyses can be made, yielding upper state B, C, and B constants, all four bands exhibit irregular subband origins such that in no case can an accurate vibrational origin be determined. The rather close-lying perturbing levels can almost certainly be identified but no strictly quantitative interpretation of the perturbations can be made. Studies of the spectra of H213C12CO and H212C13CO confirm the origin of these perturbations. They are particularly acute in the cases of ν3 (∼ 1387 cm−1), in Fermi resonance with ν8 + ν9, and of ν4 (∼ 1116 cm−1), in Fermi resonance with ν5 + ν6, and also 2ν5 and 2ν6. Indeed, in the latter case, it is debatable whether the observed band should be called ν4 at all. On the other hand, analyses of the 2ν8 overtones of the CH2 rocking fundamentals in the isotopic ketenes show little evidence of the effects of vibrational perturbations. The four fundamentals of dideuteroketene have also been studied; only ν1 would appear to be essentially unperturbed.

Experimental and ab initio equilibrium structure of trans-diazene HNNH

Abstract The experimental rz and re structures of trans-diazene have been calculated using the experimental ground state rotational constants of N2D2, N2D2 and N2DH and a newly determined harmonic force field. For comparison the structure has been calculated ab initio at the MP2, CCSD and CCSD(T) levels using a polarized triple-zeta basis set. The average geometry is determined to be: r z ( NH ) = 1.041(1) A , r z ( NN ) = 1.252(1) A and ∠(HNN) = 106.3(1)° and the equilibrium one: r e ( NH ) = 1.029(1) A , r e ( NN ) = 1.247(1) A and ∠(HNN) = 106.3(1)°. The Coriolis interacting bands in HNNH. DNND and HNND were reanalysed using the Coriolis constants calculated from the force field.

Studies of intermolecular vibrations in hydrogen-bonded molecular complexes for the gas phase using a synchrotron radiation source

An electron storage ring acts as an approximate point source in the far-infrared spectral region with a much greater intensity than provided by conventional black body radiators. Far-infrared synchrotron radiation is therefore suitable for high-resolution spectroscopic measurements of low-frequency vibrations for metastable molecules such as the intermolecular vibrational modes of hydrogen-bonded molecular complexes. The present paper describes the use of synchrotron radiation from the MAX-I electron storage ring, Lund, Sweden, for far-infrared measurements of several hydrogen-bonded molecular complexes in the gas phase.

Coriolis perturbations in the ν10, ν7, ν4, ν12 band system of trans-d2-ethylene

Abstract The effect of a -, b -, and c -axis Coriolis perturbations in the infrared spectrum of the band system ν 10 , ν 7 , ν 4 , ν 12 of trans-d 2 -ethylene has been studied at a resolution near 0.03 cm −1 . From a global analysis of this band system taking into account Coriolis resonances, spectroscopic constants for each of the vibrations are derived as well as second-order Coriolis interaction constants for ν 10 and ν 7 .

The high-resolution infrared spectrum of ethylene-d4 below 1200 cm−1

Abstract The IR spectrum of the fundamental bands ν 12 , ν 7 , and ν 10 in ethylene- d 4 at 1077, 720, and 593 cm −1 , respectively, has been recorded with a resolution of ca. 0.03 cm −1 . The three bands have been analyzed simultaneously from a model taking into account all first-order Coriolis resonances within the tetrad ν 12 , ν 4 , ν 7 , ν 10 . In addition, second-order a -Coriolis resonance between ν 7 and ν 10 and third-order b -Coriolis resonance between ν 4 and ν 10 , which both give rise to local perturbations, have been included. The analysis yields a set of spectroscopic constants for the three bands. Furthermore, the band center and the rotational constants for the inactive ν 4 level have been determined from its perturbing effect on ν 7 and ν 10 . A ground state combination difference analysis of ν 12 and ν 7 yields the following ground state rotational constants: A 0 = 2.441656(20) cm −1 , B 0 = 0.734913(6) cm −1 , and C 0 = 0.563517(6) cm −1 .

The high-resolution infrared spectrum of the ν7 + ν8 and ν4 + ν8 bands of trans-d2-ethylene

Abstract The IR spectrum of the combination bands ν7 + ν8 of trans-d2-ethylene at 1586 and 1846 cm−1, respectively, has been recorded with a resolution of ca. 0.005 cm−1. In both bands only the type a component is observed. The analysis of a number of local perturbations identified in many of the Ka subbands of ν7 + ν8 according to a model taking into account several types of Coriolis interactions within the tetrad ν8 + ν10, ν7 + ν8, ν6 + ν10, ν6 + ν7 has led to the determination of upper state spectroscopic constants for ν7 + ν8 and a number of interaction constants. Even though none of the perturbing levels have been observed, some parameters, including band centers, have been determined for the levels ν8 + ν10 and ν6 + ν10 from their perturbing effects on ν7 + ν8. The ν4 + ν8 band appears less perturbed. Spectroscopic constants for this band have been obtained from a model including Coriolis resonances with the unobserved ν6 + ν7 and ν4 + ν6 bands. From an analysis of ground state combination differences from ν7 + ν8, ν4 + ν8 and the type b component of the unpublished CH-stretching fundamental ν9 the following ground state rotational constants have been derived: A0 = 3.486176(8) cm−1, B0 = 0.8329591(14) cm−1, C0 = 0.6706399(14) cm−1.

Coriolis perturbations in the infrared spectrum of the ν4 + ν7 and ν7 + ν8 bands of cis-d2-ethylene

Abstract The infrared spectra of the ν 4 + ν 7 and ν 7 + ν 8 type a combination bands of cis -C 2 H 2 D 2 at 1816 and 1599 cm −1 , respectively, have been recorded with a resolution of ca. 0.03 cm −1 . Both bands are locally perturbed by a number of Coriolis resonances with nearby binary combination levels. From a model taking into account Coriolis interactions with ν 6 + ν 8 and ν 6 + ν 7 spectroscopic constants have been obtained for ν 4 + ν 7 . For the analysis of ν 7 + ν 8 Coriolis resonance with ν 2 has been included. None of the perturbing levels have been observed. However, from the perturbation effect alone the band centers for ν 6 + ν 8 , ν 6 + ν 7 , and ν 2 have been located.

The infrared spectrum of gaseous cis-d2-ethylene below 1400 cm−1

Abstract Three infrared active fundamental bands of cis-d 2 -ethylene have been studied at a resolution of ca. 0.030 cm −1 : the type- c band ν 7 and the two type- a bands ν 6 and ν 12 . From a simultaneous analysis of infrared ground state combination differences due to ν 7 together with microwave measurements, a set of ground state rotational and centrifugal distortion constants has been obtained. For all three bands upper state spectroscopic constants are determined, and perturbations are identified. The K ′ a = 4 level in ν 7 is perturbed locally by a higher order c -Coriolis resonance with ν 8 . ν 6 is globally perturbed by first-order a -Coriolis resonances with ν 4 and ν 8 . For ν 12 a higher order c -type Coriolis resonance with 2 ν 10 is of importance for several K a levels, and the constants for ν 12 have been obtained taking this interaction into account. In addition, a Coriolis resonance parameter and some constants for 2 ν 10 have been determined.