Svein Samdal

Structure and barrier of internal rotation of biphenyl derivatives in the gaseous state: Part 1. The molecular structure and normal coordinate analysis of normal biphenyl and pedeuterated biphenyl

Abstract The structures of the title compounds have been determined in the gaseous state. Both static and dynamic models have been applied. The structure parameters are found to be: r(C1C1′) = 1.507(4) and 1.489(4). r(C1C2) = 1.404(4) and 1.403(6), r(C2C3)= 1.395(5) and 1.396(8), r(C3C4) = 1.396(5) and 1.398(13), r(CH) = 1.102(2) and r(CD) = 1.095(2), ∠C2C1C6 = 119.4(4) and 117.9(4), ∠/C1C2C3 = 119.4(4) and 121.3(4) respectively for C12H10 and C12D10. Distances, re, are in A and angles, ∠α, in degrees. Both molecules are non planar with a torsional angle equal to 44.4(1.2) and 45.5(1.6) for C12H10 and C12D10 derived from the dynamic model using the potential function V(o) = (V2/2)(1 − cos 2o) + (V4/2)(1 − cos 4o) where V2 = 0,5(1.1) and -0.6(1.9) kJ mol−13 V4 = −6.2(2.3) and −9.5(3.6) kJ mol−1 for C12H10 and C12D10, respectively. The barriers at O° are 6.0(2.1) and 9.9(3.0) kJ mol−1, and at 90° 6.5(2.0) and 9.2 (2.6) kJ mol−1, respectively for C12H10 and C13D13. Uncertainty is one standard deviation from least-squares refinement using a diagonal weight matrix. With the exception of the torsional angles all the geometrical parameters for C12H10 and C12D10 are the same both comparing the two compounds and the results obtained in the gas phase and in the crystal, the experimental errors taken into consideration.

Molecular conformation of 2,2'-bithiophene determined by gas phase electron diffraction and ab initio calculations

Abstract Gas phase electron diffraction was performed at 97–98°C on 2,2′-bithiophene and the experimental data show the existence of two conformations, anti -like and syn -like, with torsional angles of 148(3) and 36(5)° and conformational weights 56(4) and 44(4)%, respectively, which are in agreement with ab initio calculations using standard 3-21G ∗ and 6-31G ∗ basis sets.

Potential for rotation about C(sp2)-O and C(sp2)-S bonds: electron diffraction results for CH2CH-OCH3 and CH2

Abstract Electron diffraction of gaseous methyl vinyl ether and methyl vinyl sulphide show that both compounds exist as a mixture of two conformers, one of these is the syn form with a planar heavy atom skeleton. For methyl vinyl ether the second conformer may also have a planar skeleton ( anti form), but a gauche form with a torsional angle close to 180° cannot be ruled out. For the sulphide a gauche form is found. Structural parameters are given in Table 2. Ab initio calculations gave energy minima for syn and anti forms for CH 2 CH-OH and for syn and gauche forms for CH 2 CH-SH.

Syn—gauche equilibrium of methyl vinyl sulphide studied by the combined use of microwave and raman spectroscopy, electron diffraction, and molecular force field and ab initio calculations

Abstract Gaseous methyl vinyl sulphide has been reinvestigated using electron diffraction. Published results from ab initio calculations and microwave spectroscopy have been incorporated in the analysis. Raman spectra have been recorded at different temperatures, and a Urey—Bradley force field has been determined. A dynamic model has been used to account for the large amplitude motion due to the internal rotation. The relative merits of using such a model rather than one involving a small amplitude motion have been discussed. The energy difference, Δ E 0 , between the syn and gauche (or a quasi-planar anti ) conformation is determined to be 1.9(2) kcal mol −1 and 1.0(3) kcal mol −1 , using a large and a small amplitude model, respectively. However, both models contain assumptions which affect the determination of Δ E 0 , but not the magnitude of the standard deviations given in parentheses, which are derived from the least-squares analysis. Including these assumptions, Δ E 0 is most likely to be in the range 1.0–2.0 kcal mol −1 . The most important structural parameters for the syn conformation are: r a (CC) = 1.343(1) A, r a (C sp 2 S) = 1.759(8) A, r a (C sp 3 S) = 1.795(8) A, r a (C sp 2 H) = 1.088(9) A, r a (C sp 3 H) = 1.106(8) A, ∠ α CCS = 127.5(7)° and ∠ α CSC = 102.1(5)°. The torsion angles for the syn and gauche conformations are 0° and 135.8(6.5)°, respectively.

The molecular structure of decamethylferrocene studied by gas phase electron diffraction. Determination of equilibrium conformation and barrier to internal rotation of the ligand rings

Abstract The molecular structure of decamethylferrocene, (η-C 5 Me 5 ) 2 Fe, has been determined by gas phase electron diffraction. The FeC and C(Cp)C(Cp) bond distances, 2.064(3) and 1.439(2) A, respectively, are indistinguishable from those in ferrocene, Cp 2 Fe. But, while the equilibrium conformation of gaseous Cp 2 Fe is eclipsed ( D 5h ), the equilibrium conformation of (C 5 Me 5 ) 2 Fe is staggered ( D 5d ) with a barrier to internal rotation of the ligand rings V 5 = 1.0(0.3) kcal mol −1 . And while the CH bonds in Cp 2 Fe are bent about 4° out of the plane of the C 5 ring towards the metal atom, the C(Cp)C(Me) bonds in (C 5 Me 5 ) 2 Fe are bent 3.4(0.5)° out of the plane in the opposite direction.

The molecular structure of high- and low-spin 1,1′-dimethyl-manganocene determined by gas phase electron diffraction

Abstract The electron diffraction pattern of 1,1′-dimethyl-manganocene has been recorded from s = 3.00 to 42.00 A −1 . The gas is found to contain two geometrically distinct species. The most abundant species, mole fraction x = 0.62(4), has a MnC bond distance R(MnC) = 2.433(8) A and vibrational amplitude (MnC) = 0.111(8) A. By comparison with the structure of the essentially high-spin complex (C 5 H 5 ) 2 Mn where R(MnC) = 2.38 A, it is concluded that the most abundant species is in the high-spin, 6 A 1 g , state. The less abundant species, x = 0.38(4), has an MnC bond distance R(MnC) = 2.144(12) A and vibrational amplitude (MnC) = 0.160(16) A. This species is assumed to be in a low-spin, 2 E 2 g , state. The large MnC vibrational amplitude of the low-spin species is consistent with the existence of a dynamic Jahn-Teller effect involving the ring tilting modes.

The molecular structure of benzene derivatives part 1. 4-Fluorobenzaldehyde by joint analysis of gas electron diffraction, microwave spectroscopy and ab initio molecular orbital calculations

Abstract The molecular structure of gaseous 4-fluorobenzaldehyde has been determined by a joint analysis of gas electron diffraction data, rotational constants from microwave spectroscopy, and constrained by results from ab initio calculations. The ab initio calculations have been performed at the HF 6-311 G ∗∗ and MP 2 6-31 G ∗ levels of theory. The planar Cs symmetry structure was found to be the only stable conformation. The torsion of the formyl group has been treated as a large-amplitude motion. The most important structure parameters (rg) from the joint analysis with estimates of 2σ (in parentheses) were: ( CC ) mean = 1.397(1) A , CF = 1.331(7) A , CC (= O ) = 1.488(7) A , CO = 1.195(5) A ,

The molecular structure of gaseous monobromobenzene

Abstract The molecular structure of gaseous monobromobenzene has been studied by the electron diffraction method. The molecular geometry was determined by a conjoint analysis based on electron diffraction intensities and microwave rotational constants, assuming C 2v molecular symmetry. The angular distortion of the benzene ring mainly affects the internal angle at the ipso carbon atom: this angle is determined to be ∠ α (C 2 C 1 C a ) = 121.5(4)° which, as expected for an electronegative substituent, is significantly larger than 120°. The other geometrical parameters are: r a (C 1 Br) = 1.898(1) A, r a (C 1 C 2 ) = 1.394(3) A, r n (C 2 C 1 ) = 1.396(5) A, r a (C 2 C 1 ) = 1.394(7) A r a (C 2 H 2 ) = 1.097(3) A, r a (C 2 H 2 ) = 1.086(3) A, r a (C 4 H 9 ) = 1.085(3) A, ∠ α C 3 = 119.0(7)°, ∠ α C 1 C 2 H 7 = 121.7(1.1)° and ∠ α C 4 C 3 H 3 = 120.5(1.1)°. The r o α (CH) bond lengths are assumed to be equal and are refined in one group. Parenthesized values are one standard deviation from the least-squares refinement.

MICROWAVE SPECTRUM, CONFORMATION, AB INITIO CALCULATIONS, BARRIER TO INTERNAL ROTATION AND DIPOLE MOMENT OF PROPIONAMIDE

Abstract The microwave spectrum of propionamide has been investigated in the 21.4–39 GHz spectral range. One conformer has been assigned. This rotamer has a C s equilibrium conformation and the methyl group is syn to the carbonyl group. The dipole moments are μ a = 2.121(20), μ b = 11.66(10) and μ tot. = 11.85(10) × 10 −30 C m. The nuclear quadrupole coupling constants of the 14 N nucleus are χ aa = 2.2(8) and χ bb = 2.3(5)MHz. The barrier to internal rotation of the methyl group is 9.1(5) kJ mol −1 . Six vibrationally excited states of the torsion around the C2–C6 bond were assigned and this fundamental frequency was found to be 45(7)cm −1 . These vibrationally excited states were used to approximate the potential function for torsion near its bottom as V = 7.0 (〈 z 4 〉 + 2.0〈 z 2 〉) cm −1 . This function implies that there is no (or a very small) potential hump at the heavy-atom planar conformation. The first vibrationally excited state of the methyl-group torsion was also assigned. Ab initio computations have been made at the 6–31G ∗ , 6&3ndash;11G ∗∗ , 6–311+ G ∗ , MP 2 6–311 ++ G ∗∗ and MP 3 6–311 ++ G ∗∗ levels of theory. The ab initio calculations imply that there is no second stable form of propionamide. These computations also predict a non-planar heavy-atom conformation at all levels of theory. The Hartree-Fock calculations predict a very small barrier to planarity which is not in complete disagreement with the microwave data. However, the computations involving electron correlation are in obvious disagreement with the experimental findings because a rather high barrier to planarity of 1.6 kJ mol −1 is predicted.

Structure and barrier of internal rotation of biphenyl derivatives in the gaseous state: Part 3. Structure of 4-fluoro-, 4,4′ -difluoro-, 4-chloro - and 4,4′-dichlorobiphenyl

Abstract Gas-phase electron diffraction structures of the four title compounds have been determined. The structure parameters were found to be: 4-Fluorobiphenyl: r(CF) = 1.365(4), r(CC)ave = 1.396(1), r(C1C1′) = 1.497(3), r(CH)ave = 1.097(2), ∠C6C1C2 = 118.8(5), ∠C1C2C3 = 121.8(3), ∠C6′C1′C2′ = 118.4(6), ∠C1′C2′C3′ = 120.8(2) 4-Chlorobiphenyl: r(CCl) = 1.735(2), r(CC)ave = 1.397(1), r(C1C1′) = 1.480(4), r(CH)ave = 1.099(2), ∠C6C1C2 = 118.6(6), ∠C1C2C3 = 121.3(5), ∠C6′C1′C2′ = 117.4(6), ∠C1′C2′C3′ = 120.7(3) 4,4′-Difluorobiphenyl: r(CF′) = 1.349(2), r(C1C2) = 1.413(3), r(C2C3) = 1.393(3) r(C3C4′) = 1.387(2),r(C1C1′) = 1.483(4), r(CH)ave = 1.085(3), ∠C6C1C2 = 117.8(3) ∠C1C2C3 = 121.3(2) 4,4′-Dichlorobiphenyl: r(CC1) = 1.736(1), r(C1C2) = 1.405(4), r(C2C3) = 1.385(3), r(C3C4) = 1.401(3), r(C1C1′) = 1.494(5), r(CH)ave = 1.093(3), ∠C6C1C2 = 118.1(3), ∠C1C2C3 = 121.4(3) Distances, ra, are given in Angtroms and angles, ∠α, in degerees refering to the dynamic model. Both static and dynamic models have been applied in the investigation of the large amplitude motion about the inter-ring CC bond. All title compounds are non planar. The dynamic model (applied potential function V(o)) = 1 2 V3(1 - cos 2o) + 1 2 V4 (1 - cos 4o) gave dihedral angles 44.8(0.8), 44.1(1.1), 45.0(1.0) and 45.2(1.5)°, and the Fourier coefficients V1 and V4 equal to 0.3(1.3) and -11.0(3.1) 0.9(1.2) and -7.2(2.8), 0.0(1.3) and -9.5(2.7), -0.3(1.8) and -8.3(3.1) kJ mol−1 respectively for 4-fluoro-, 4-chloro-4,4′-difluoro- and 4,4′-dichlorobiphenyl. The numbers in parentheses are one standard deviation as given by least-squares refinements using a diagonal weight matrix.