C.J. Nielsen

The vibrational spectra, molecular structure and conformation of organic azides: Part IV. An ab initio study of hydrazoic acid, azidomethane, azidoethane, azidoethene and azidomethanal

Abstract Fully optimized geometries have been calculated for the title compounds at the Hartree—Fock SCF level and compared with existing experimental data. A basis set of double zeta quality has been employed. For hydrazoic acid, a calculation with a larger basis set, expected to give results near the Hartree—Fock limit, has also been performed. All of the calculations show the azide group to be slightly bent with a trans configuration around the central NN bond. Azidoethane is predicted to exist in two conformations, gauche (71°) and anti, with a negligible energy difference of 0.26 kJ mol−1 between them. Azidoethene and azidomethanal both prefer the syn orientation of the azide group with respect to the CC or CO bonds, the computed energy difference between the anti and syn conformations being 3.31 and 30.3 kJ mol−1 respectively. The barrier to rotation around the CN bond has been calculated to be 3.75 kJ mol−1 in azidomethane while in azidoethane it was 3.30 and 9.40 kJ mol−1 in the eclipsed anti-clinal (120°) and syn positions, respectively. Complete harmonic force fields and dipole moment derivatives have been calculated for hydrazoic acid, azidomethane and for the two stable conformations of azidoethane. For azidoethane and azidomethanal only the azide part of the harmonic force field has been calculated. The theoretical harmonic force fields have been modified through scaling by a least squares refinement to the observed wavenumbers of hydrazoic acid, azidomethane and azidoethane (anti and gauche). Infrared vapour phase intensities have been calculated and theoretical spectra are presented for azidomethane and azidoethane.

The vibrational spectra, molecular structure and conformations of organic azides. I. A survey

Abstract A number of organic monoazides (RN 3 ) have been synthesized in which R is: (1) a saturated group, CH 3 , C 2 H 5 , nC 3 H 7 ; (2) an olefinic group, allyl, butadiene; (3) an acetylenic group, NCCH 2 , HCCCH 2 , CH 3 CCCH 2 . Two additional unsaturated diazides (CH 2 C(N 3 )C(N 3 )C(N 3 )CH 2 and N 3 CH 2 CCCH 2 N 3 ) were prepared. The compounds (most of them very explosive) were studied by IR and Raman spectroscopy in the liquid, in solution and in the solid state, and by matrix isolation technique in IR. The spectra were interpreted in terms of one or in some cases two or more conformers and assigned with the aid of normal coordinate analysis. UV photolysis experiments in nitrogen matrices at 12 K were carried out and the reactions monitored by FTIR. The intermediate products could in some cases be identified as imines. Six of the azides were investigated by gaseous electron diffraction and the molecular structures established. The azide group was situated gauche to the hydrocarbon skeleton in NCCH 2 N 3 , HCCCH 2 N 3 and CH 3 CCCH 2 N 3 . In the butadienes CH 2 C(N 3 )CHCH 2 and CH 2 C(N 3 )C(N 3 )CH 2 the azide group was syn to the adjacent CC bond, while in H 2 CCHCH 2 N 3 at least two conformers were detected. Model calculations on the smaller azides by ab initio quantum chemical methods were used to establish trends in the geometry and force fields of the azide group.

The vibrational spectra, molecular structure and conformation of organic azides: Part III. 2,3-Diazido-1,3-butadiene

Abstract A sample of 2-azido-1,3-butadiene was synthesized from 4-bromo-1,2-butadiene and tetramethylguanidinium azide. Although the sample is highly explosive, we succeeded in making a structure determination by gaseous electron diffraction. IR spectra of the vapour, of the matrix isolated species in argon at 15 K, and of an amorphous and crystalline solid at 90 K were recorded. A Raman spectrum of the liquid, including semiquantitative polarization data, was obtained at 240 K. The title compound was found to be planar with the CNN angle 117° oriented syn to the adjacent CC double bond, the NNN angle was ca. 177° oriented anti to the CN bond. The following bond distances (ra) were obtained: NN(N), 114.3; NN(C), 125.3; CN, 143.4; CC, 135.0; and CC, 146.7 pm. No additional conformers were observed in the vapour, liquid, amorphous or crystalline states.

Molecular structure and conformational equilibrium of gaseous thiophene-2-aldehyde as studied by electron diffraction and microwave, infrared, Raman and matrix isolation spectroscopy

Thiophene-2-aldehyde has been investigated by microwave, IR and Raman spectroscopy and by electron diffraction of the vapour. The compound was also isolated in argon and nitrogen matrices and studied by IR spectroscopy. Two conformers were identified, a more stable planar isomer with the S atom syn to the 0 atom and a less stable planar (or near planar) anti form. Assuming that the geometry of the two forms differs only in the S-C-C=0 torsion angle and, assuming the thiophene ring to have C,, symmetry, the electron diffraction study gives the following result for some of the distances (ra) and angles (&): r(C-H) = 1.114(20) A, r(C=O) = 1.224(7) a, r(C-S) = 1.717(4) a, r(C=C) = 1.375(7) A, r(C-C) (in th e ring) = 1.431(15) A, r(C-COH) = 1.466(16) a, LC=C-COH = 126.4(1.3)“, f.C=C-S = 111.8(4)‘, LC=C-H = 129.2(4.0)“, IC--C=O = 123.7(g)“, and Lo (S-C-C=0 torsion angle in the anti conformer) = 158.5 (23.1): At 94°C the observed amount of the conformer with 0 and S syn was 80.5(7.9)%, and the syn conformer had a r.m.s. torsional amplitude of vibration of 7 = 17.2(13.6)“. Assuming ASo = 0, the obtained amount of syn corresponds to AH” = 4.3 5 1.6 kJ mol-’ . The matrix isolation study included the use of a heatable nozzle, which made it possible to trap different conformational equilibria in the matrices. By comparing absorbances obtained for different temperatures, the enthalpy difference between the conformers was estimated as 4.1 f 0.4 kJ mol-‘, in very good agreement with the electron diffraction result. The matrix data support the extensive IR and Raman study of the other phases (vapour, liquid, solution and solid). In the crystalline solid the preferred conformation was found to be syn. The microwave investigation shows the planarity of the syn isomer, whereas the anti form was not detected. The spectral data have hence been interpreted in terms of C, symmetry, and a normal coordinate analysis has been carried out for both conformers. The final structural refinement was based upon electron diffraction intensities in combination with the microwave rotational constants. The vibrational amplitude parameters applied were derived from the force field calculations.

The molecular structure, conformations and vibrational spectra of 2,2-di(chloromethyl)-1,3-dichloropropane and 2,2-di(bromomethyl)-1,3-dibromopropane

Abstract The IR and Raman spectra of 2,2-di(chloromethyl)-1,3-dichloropropane (C(CH2Cl)4) and 2,2-di(bromomethyl)-1,3-dibromopropane (C(CH2BR)4) were recorded as melts and as solutes in various solvents. Spectra of the solids were observed at various temperatures between the melting points and 90 K. High pressure IR spectra (0–20 kbar) of the compounds were recorded between 300 and 450 K. The crystal structures of both compounds were determined by X-ray crystallographic measurements of single crystal at ca. 130 K. In the crystalline state both compounds exist in the D2d conformer, whereas in the melt and in solution an additional conformer probably of symmetry S4 was assigned supported by force constant calculations. Unlike neopentane and various chlorinated neopentanes with one, two or three chlorine substituents, no plastic crystalline phase was detected for the title compounds. The structure of both molecules were disordered with two molecules in the monoclinic unit cell (P21/n).

The vibrational spectra, molecular structure and conformation of organic azides—IX. Azidoethane☆

Abstract A safe method for the synthesis of azidoethane from ethylbromide is given and 1H and 13C NMR data are reported. The i.r. and Raman spectra of azidoethane have been 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 and of the gauche conformer in the crystalline solid. Matrix isolation studies reveal the gauche conformation to be the more stable in argon and in nitrogen matrices and probably also more stable in the vapour. The enthalpy difference between the conformers is calculated to be ΔH0a→g (N2 matrix) ≈ ΔH0a→g (vap.) = −0.56(10) kJ mol−1, and the barrier to rotational isomerism (anti → gauche) as 9.0(10) kJ mol−1 in the nitrogen matrix and less than 6 kJ mol−1 in the argon matrix. Careful Raman studies of the liquid at 140–290 K reveal the gauche conformation to be the more stable in the liquid phase as well, the enthalpy difference being ΔH0a→g (liq.) = − 1.15 (5) kJ mol−1. The majority of the fundamentals for both conformers have been assigned with the aid of normal coordinate calculations using previously developed scaled quantum mechanical force fields which are also presented.

The vibrational spectra, molecular structure and conformation of organic azides: Part VI. Azidoacetonitrile

Abstract Azidoacetonitrile (azidoethanenitrile) has been synthesized and the structure determined by electron diffraction from the vapour. IR spectra have been recorded of the vapour, liquid, solutions, solid at ca. 90 K and of the molecule trapped in argon and nitrogen matrices at 15 K. Raman spectra of the cooled liquid and of the amorphous and crystalline solids have also been obtained. The electron diffraction results show the conformation to be gauche around the CN bond with a dihedral angle of 52°(5) from syn and the NNN angle 173°(3) oriented anti to the CN bond. The following bond distances ( r a ) between the heavy atoms were obtained: r CN = 115.4(5) pm, r CC = 146.5(15) pm, r CN = 147.5(6) pm, r NN = 124.5(5) pm and r NN = 113.5(4) pm. Very large changes in both frequency and intensity have been observed in the vibrational spectra upon crystallization which may be due to intermolecular associations formed by the azido group but which can also be explained by a change of the molecular conformation from only gauche in the vapour and liquid phases to anti in the crystal phase.

Vibrational spectra, molecular structure and conformation of organic azides: Part VII. Azido-2-butyne

Abstract A new organic azide, azido-2-butyne, has been synthesized from bromo-2-butyne and tetramethylguanidinium azide and the 1 H and 13 C NMR data are reported. The structure has been determined by electron diffraction from the vapour. IR spectra have been recorded of the vapour, of the liquid, of the amorphous and crystalline solids at ca. 90 K and of the molecule trapped in nitrogen matrices at 15 K. Raman spectra of the cooled liquid and of the amorphous and crystalline solids at 90 K have also been obtained. The spectroscopic results indicate that the molecule exists in only one conformation in all the states of aggregation and the electron diffraction results show this conformation to be gauche around the CN bond with a dihedral angle of 37(10)° from syn and the NNN angle 174(5)° oriented anti to the CN bond. The following bond distances ( r a ) and angles (∠ α ) between the heavy atoms are obtained assuming a linear butyne skeleton and r CCH 3 = 146.0 pm : r CC = 120.8(6) pm, r CCH 2 N 3 = 146.8(5) pm, r CN = 147.4(15) pm, r NN = 124.0(6) pm, r NN = 114.2(5) pm, ∠ NNC = 116.5(14)° and ∠ NNC = 113.7(16)°.

The vibrational spectra, molecular structure and conformation of organic azides: Part VIII. 3-Azidopropene (allylazide)☆

Abstract 3-Azidopropene (CH 2 CHCH 2 N 3 ) has been synthesized and the structure and conformational composition studied by electron diffraction from the vapour. IR spectra have been recorded of the vapour, of the liquid and of the amorphous solid at ca. 90 K. Raman spectra of the cooled liquid and of the amorphous and crystalline solids at 90 K have also been obtained. Analysis of the electron diffraction data reveals a multiplicity of indistinguishable solutions to the conformational composition consisting of variable amounts of the three conformers SG , GG and GG′ , the letters referring to the conformation around the CC and CN bonds, respectively. As extremes, the electron diffraction data could be interpreted by either a mixture of only two conformers, 73(15)% GG and 27% SG , or a mixture consisting of equal amounts of GG and GG′ , 87(20)% in total, and 13% SG . Assuming only the GG and SG conformers to be present in the vapour, a transoid arrangement of the azide chain, r CC =150.8 pm, ∠ CCH =120.8° and torsional angles τ CC =0, 120° the following structural parameters ( r a , ∠ α ) are derived for the GG form: r NN =113.8 (4) pm, r NN =123.6 (5) pm, r CN =147.5 (15) pm, r CC =133.1 (7) pm, ∠ NNN =174(5)°, ∠ CNN =115.1(14)°, ∠ NCC =111.8(34)°, ∠ CCC =121.6(30)° and τ CN =60(10)°. The vibrational spectra are interpreted in terms of two conformers, GG and SG , present in the vapour and the liquid and, based upon normal coordinate calculations, of the SG conformer in the crystalline solid. The normal modes of vibration for the five distinct conformations of 3-azidopropene are calculated using transferred scaled quantum mechanical force fields for propene and azidoethane.

The ab initio calculated molecular structures, force fields and vibrational frequencies of some organic azides

Abstract Double zeta basis molecular calculations were carried out on the hydrazoic acid and azidomethane molecules. The molecular structures were optimized by the gradient method and the force fields were obtained by numerical differentiation of the gradient vector. The computed harmonic force fields and vibrational frequencies were compared with experimental values.