Grete Gundersen

On the molecular structure of dimethylaluminium fluoride tetramer, [(CH3)2AIF]4

(CH3)2AlF has been studied by gas phase electron diffraction. The compound is tetrameric under the experimental conditions. The main molecular parameters are R(AlC) 1.947 (4), R(AlF) 1.810 (3) A, < CAlC 131.2 (1.9)°, < FAlF 923 (1.2)° and < AlFAl 146.1 (2.6)° the Al4F4 ring being puckered. The factors determining the degree of association of compounds of the type R2AlX (X = F,OR′, NR′2, Cl, SR′ and PR′2) are discussed, and it is suggested that Pitzer strain may be a significant factor in some cases.

Molecular structure of gaseous methyl borate, B(OCH3)3

Abstract The molecular structure of methyl borate has been investigated by electron diffraction from the vapour. The data were found to be consistent with a B(OC) 3 skeleton of C 3h , symmetry. The principal molecular parameters are: r a (B-O) = 1.367(4) A, r a (O-C) = 1.424(5) A, ∠BOC = 121.4(0.5)°, φ (the HCBO-dihedral angle relative to 0° for the syn form) = 23.6(5.6)°. l(B-O) = 0.046(4) A, and l(O-C) = 0.044(4) A, as determined from geometrically consistent r ga -refinements. Parenthesized values are 2σ where estimates of systematic uncertainties are included for the distance parameters. Root-mean-square amplitudes of vibration calculated from spectroscopic data are also given.

Persistent phosphinyl radicals from a bulky diphosphine: an example of a molecular jack-in-the-box

The structure of the phosphinyl radical, ṖR2 [R = CH(SiMe3)2], has been determined by gas-phase electron diffraction (GED) together with ab initio molecular orbital calculations, and that of the corresponding diphosphine, (PR2)2, has been established by X-ray crystallography; the diphosphine behaves as an energy storage reservoir.

Vibrational spectra, molecular structure and conformation of gaseous 3-thiocyanatopropyne (propargylthiocyanate)

Abstract 3-Thiocyanatopropyne has been prepared and the IR and Raman spectra recorded in the region 4000-40 cm −1 and interpreted in terms of two conformers, anti and gauche , present in the vapour and in the liquid. Electron diffraction shows that the title compound exists as a mixture of two conformers in the vapour phase: 55(6) % gauche with a dihedral angle θ CCSC =53(5)° and 45(6) % anti with θ CCSC =180° (fixed) at 303 K. Neglecting conformational entropy differences other than the statistical weight of two for gauche , this corresponds to an energy difference of 1.24 kJ mol −1 , anti being marginally the low-energy form. The structural parameters for the two conformers are assumed to differ only by their torsion angle values, and some of the more important bond distances and angles are ( r a and ∠ α ): I CN =116.8 (4), r CC =120.7 (5), r CC =144.4(4), r C( sp )S =168.9(3) and r C( sp 3 )S =183.6(3) pm; and ∠CCS=112.7(6)° and ∠CSC=97.4(10)°. The r.m.s. torsional angle amplitudes are δ gauche =13(4)° and δ anti =20(7)°.

The vibrational spectra of cis- and trans-1-chloro-1,3-butadiene

Abstract The i.r. spectra of cis - and trans -1-chloro-1,3-butadiene as vapours, in solution and as amorphous and crystalline solids at −180°C were recorded in the regions 4000-50 and 4000-200 cm −1 , respectively. Raman spectra, including polarization measurements of the neat liquids were obtained, and additional spectra of the amorphous and annealed crystalline solids were recorded. A common 30 parameter valence force field was derived for the two monochlorobutadienes in which the initial force constants were transferred mainly from butadiene and chloroethylene. Adjustments of certain force constants improved the fit between the calculated and observed frequencies and between the calculated amplitudes of vibration and those determined from preliminary electron diffraction studies. The vibrational fundamental frequencies for the present monochlorobutadienes have been assigned in terms of C s molecular symmetry on the basis of infrared vapour contours, Raman polarization measurements and the results of the force constant calculation.

On the molecular structures of the prevailing anti conformers of gaseous cis-1 and trans-1-chloro-1,3-butadiene

Abstract The molecular structures of the title compounds have been investigated by ab initio MO calculations and by analyses of room-temperature electron diffraction data alone, as well as by conjoint use of these data and literature ground-state rotational constants of eleven isotopic species. Models for the structures were specified in terms of geometrically consistent r α -parameters under the assumption of C s molecular symmetry. Literature molecular force fields were used to calculate vibrational amplitude parameter quantities needed for vibrational corrections and for making the three methods compatible. Results of the computed structures were incorporated into the structure analyses so as to reduce the geometry-defining variables for the hydrogen positions from ten to four, and ultimately to constrain the difference parameter Δ a (CC) = r a (C 1 C 2 ) - r a (C 3 C 4 ), to 0.37 and 0.40 pm respectively for the cis and trans isomer. The final structures obtained by conjoint consideration of computational, electron diffraction and spectroscopic evidence then have the following principal distances ( r a in pm) and angle (∠C α in degrees) with estimated standard deviations in parentheses: the cis isomer, r (C 1 C 2 ) = 134.2(2), r (C 3 C 4 ) = 134.5(2), r (C 2 C 3 ) = 146.6(3), r (C-Cl) = 173.0(2), ∠C 1 C 2 C 3 = 125.5(2), ∠C 4 C 3 C 2 = 122.3(4), ∠C 2 C 1 Cl = 123.9(2); the trans isomer, r (C 1 C 2 ) = 134.0(2), r (C 3 C 4 ) = 134.4(2), r (C 2 C 3 ) = 146.1(3), r (CCl) = 172.8(2), ∠C 1 C 2 C 3 = 122.5(4), ∠C 4 C 3 C 2 = 123.3(3), ∠C 2 C 1 Cl = 122.5(2). It is shown that the computational results and the GED structures are not in conflict with the rotational constants used to obtain the literature r s -structures which are however not in good agreement with the present results.

Vibrational spectra of trans,trans-1,2,3,4-tetrachloro-1,3-butadiene; and the molecular structure of trans,trans-1,2,3,4-tetrachloro-1,3-butadiene and of hexachloro-1,3-butadiene (a reinvestigation) determined by gas-phase electron diffraction

Abstract Recorded infrared and Raman spectral data for trans,trans -1,2,3,4-tetrachloro-1,3-butadiene were interpreted in terms of C 2 molecular symmetry. Force-fields established on the basis of these data and literature spectral data for hexachloro-1,3-butadiene were used to calculate vibrational amplitude quantities and correction terms introduced in the structure analyses of the gas-phase electron-diffraction data. The geometrical parameters ( r a , ∠ α ) were for trans,trans -1,2,3,4-tetrachloro-1,3-butadiene: r (CH) = 110.0(fixed), r (CC) = 134.3(5), r (CC) = 148.1(8), r (CCl) = 172.5(3) pm, ∠(C1=C2–C3) = 125.7(4), ∠(C3-C2-Cl) = 115.6(4), ∠(C2=C1-Cl) = 122.4(5), ∠ (C2=C1-H) = 124.0(fixed) and φ(CCCC) = 76.6(23)°; and for hexachloro-1,3-butadiene: r (CCl) = 171.6(3), r (CC) = 134.1(5), r (CC) = 148.5(9)pm, ∠(C1=C2–C3) = 122.6, ∠(C3-C2-Cl) = 115.8(4), ∠(C2=C1-Cl) = 122.50(10), ∠(C2=C1–C17) - ∠(C2=C1–C16) = −0.9(6) and φ(CCCC) = 89(3)°. The last set of parameters is consistent with previously published GED results, with the exception of the torsional angle which was found to be sensitive to assumptions and refinements of vibrational amplitude quantities. The CCCC root-mean-square torsional angle amplitudes (δ) and torsional angles (φ) determined by dynamic models of the molecules were δ = 16.3(16)° and φ=77.8(20)°; and δ=12.0(14)° and φ=83.5(19)°. Molecular mechanics calculations confirmed the synclinal conformations of the molecules and a wider torsional potential (larger δ-value) for the tetrachloro derivative which was also found to have lower torsional barriers than hexachloro-1,3-butadiene.

The molecular structure and conformational behaviour of 2-chloro-1,3-butadiene (chloroprene) studied by gas-phase electron diffraction and ab initio MO calculations

Abstract MO calculations establish a global minimum for chloroprene at the planar anti conformation (θ 1 = 180°). A subsidiary minimum appears at θ 2 = 40°, 8.3 kJ mol −1 above anti and 4.2 kJ mol −1 below planar syn with an rotational barrier of 20 kJ mol −1 at about 105°. Analyses of GED data for chloroprene at 655,565, and 298 K gave, in addition to a planar anti conformer (θ 1 = 180°), 25 (6), 18 (5), and 7(4)%, respectively, of a co-existing second form with θ 2 =27(11)°. These quantities correspond to Δ E = E 2 - E 1 =6.4(12) kJ mol −1 and Δ S c = S 2 – S 1 = − 6(3) J K −1 mol −1 . The value of θ 2 was determined from the 655 -K data using a flexible backbone structure upon rotation as extracted from the MO results. Rigid rotation gave θ 2 =41 (8)°, Δ E = 6.8(13) kJ mol −1 , and Δ S c = -5(3) J K −1 mol −1 . Some MO constraints were used in defining the positions of hydrogen atoms and for a difference r (C1C2) - r (C3C4) parameter. Important geometrical parameters ( r a ,∠ α ) of anti chloroprene as obtained in such refinements based on 298-K GED data and literature rotational constants are: r (C1C2)=134.4(2), r (C3C4)=134.4(2), r (CC)=146.9(3) and r (CCl)=174.2(2) pm; ∠CC2C = 123.5(1), ∠CC3C = 125.6 (2) and ∠CCCl = 117.2(1)°. Refinements based on the GED data alone gave the following changed parameters: r (CC)=146.0(3) pm; ∠CC2C = 123.9 (2), ∠CC3C = 126.2 (2)°.

The vibrational spectra of cis,cis-,trans,trans- and cis,trans-1,4-dichloro-1,3-butadiene

Abstract The i.r. spectra of cis,cis,trans,trans and cis,trans-1,4-dichloro-1,3-butadiene were recorded in cyclohexane solution in the region 600-40 cm−1. Raman spectra, including polarization measurements of the neat liquids were obtained. Some previously unobserved fundamentals, particularly in the low frequency region, were assigned, and a few revisions in the earlier interpretations were made. Force fields containing 21 parameters for the cis,cis and the trans,trans compounds were constructed by transferring force constants from cis- and trans-1-chloro-1,3-butadiene. By mixing the force constants from these monochlorobutadienes a 29 parameter force field was derived for the cis,trans compound. The calculated r.m.s. amplitudes of vibration and perpendicular amplitude correction coefficients are presented.

The molecular structure of diphenylchloroborane studied by gas-phase electron diffraction and ab initio MO calculations including computational studies of conformational preferences and quadratic force fields

Abstract The molecular structure of diphenylchloroborane, (C 6 H 5 ) 2 BCl, has been determined by gas-phase electron diffraction (GED) at 415 K and by ab initio MO calculations at the HF/3-21G∗ level, both being consistent with C 2 molecular symmetry. Corresponding ab initio fundamental frequencies are reported and an approximate HF/3-21G∗//HF/3-21G∗ scaled quantum mechanical (SQM) force field was established, transferring scale factors from a concurrent study of dichlorophenylborane, C 6 H 5 BCl 2 . Vibrational amplitude quantities needed in the GED structural analysis were computed from the SQM force field. Planarity of the phenyl groups and the ClBC 2 framework was assumed in addition to C 2 molecular symmetry, and the geometrical parameters determined in the GED analysis, with effects from correlation in the data and uncertainty in the s -scale (0.1%) included in the standard deviations are: r a (CH)(para) = 110.8(6) pm, r a (CC)(ortho) = 141.0(2) pm, r a (BC) = 155.5(7) pm, r a (BCl) = 178.1(8) pm, ∡ α C  B  Cl = 117.2(5)° , and ф α ( B  C ) = 29.5(1.8)° . Other geometry-defining parameters such as tilt of the phenyl groups and their distortion from the benzene structure, for example ∡ α C  C  C(ipso ) = 117.9° , were taken from the computational structure. For comparison purposes the HF/3-21G∗ computational structures are also reported for the three other molecules in the four-molecule series of (C 6 H 5 ) 3− n BCl n , n = 0, 1, 2, 3. For the three phenylboranes ( n = 2, 1, 0) there is an elongation of the BC bonds as the number of phenyl groups increases (154.4, 155.8 and 157.2 pm, with ф-values of 0, 27.7 and 33.7°), whereas for the chloroboranes ( n = 1, 2, 3) the BCl bonds become shorter (179.8, 176.9 and 174.7 pm) with an increasing number of chlorines attached to boron.