S.H. Schei

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.

3-Chloro-1-propene (allylchloride): gas-phase molecular structture and conformation as determined by electron diffraction

Abstract A gas-phase study of 3-chloro-1-propene by electron diffraction showed that the most abundant conformer was gauche with a torsional angle τ=120.8(5.4)°, relative to τ = 0 for syn from when all heavy atoms are in the same plane and the chlorine atom is eclipsing the double bond. At 20 and 90°C the amount of the gauche conformer was found to be 82(9) and 78(15)%, respectively. The least-squares refinements of the molecular structure were assisted by a few constraints taken from ab intio calculations. At 90°C the following gauche structure was obtained in terms of ra and Lα parameters: r(C=C) = 1.341(5) A, r(=CC) = 1.506(6) A, r(CCl) = 1.803(5) A, (r(CH)) = 1.081(17) A, LC=CC = 122.6(1.3)°, LCCCl = 110.0(1.0)°, LClCH = 106.6(2.4)°. Error limits are given as 2σ (σ includes uncertainty due to correlation and systematic errors in the data). The syn form was found to have 2 large root-mean-square torsional amplitude, 20–25°. In connection with the structure analysis a relatively simple valence force field was developed.

The virbrational spectra, molecular structure and conformations of organic azides: Part V. 3-Azidopropyne (propargylazide)

Abstract 3-Azidopropyne and 3-azidopropyne-1-[ 2 H] have been synthesized; the structure was determined by electron diffraction from the vapour and computed by ab initio Hartree—Fock SCF calculations. IR spectra of the vapour, of the cooled solid, of the matrix isolated species in argon and nitrogen at 13 K, and of the solid at 90 K were recorded. Raman spectra of the cooled liquid and of the amorphous and crystalline solid were also obtained. From the spectroscopic investigation it is evident that the title compound exists as only one conformer in all states of aggregation. The electron diffraction results and the theoretical calculations show unambiguously the conformation to the gauche around the CN bond with a dihedral angle of 37° (8) from syn and with the NNN angle, 169° (4), oriented anti to the CN bond. The following bond distances ( r a ) between the heavy atoms were obtained: r CC = 121.6(7) pm, r CC = 148.1(13) pm, r CN = 146.4(13) pm, r NN = 124.9(7) pm and r NN = 113.7(6) pm.

Conformation and molecular structure of 1,2-dichloropropane and 1,2-dibromopropane as determined by gas-phase electron diffraction and molecular-mechanics calculation

Abstract The conformational composition obtained for 1,2-dichloropropane at 26°C was 74% of the anti (A) conformer and 26% of the gauche (G −1 with error limits ± 7%. For 1,2-dibromopropane the conformational composition at 40°C was 85% A , 4% G − and 11% G + with error limits ±6%. Normal values of the structural parameters were determined. The results from the molecular-mechanics calculations agree with the electron diffraction data. The values of the low-frequency torsional XH 2 CC vibrations have been compared with observations form vibrational spectroscopy.

1,2,4,5-tetrafluorobenzene: molecular structure as determined by gas-phase electron diffraction and by ab initio calculations

Abstract The molecular structure of gaseous 1,2,4,5-tetrafluorobenzene has been studied by electron diffraction at 293 K, and by ab initio calculations. The benzene ring deviates only slightly from D 6h symmetry, the CC(F)C angle being 120.6(3)° in the ED investigation and 120.4° according to the ab initio calculations. The CC bonds are also practically equal, with a mean bond length of 1.388(11) A (ab initio = 1.375 A). The other independent geometry parameters are: r (CF) = 1.343(4) A, r (CH) = 1.085(48) A, and ∠FCC(H) = 119.9(5)° from the ED work. Corresponding ab initio results are 1.375 A, 1.072 A and 119.9°.

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)°.

3-Chloro-1-butene: gas-phase molecular structure and conformations as determined by electron diffraction and by molecular mechanics and ab initio calculations

Abstract Gaseous 3-chloro-1-butene has been studied experimentally by electron diffraction (ED) at 20 and 180°C, and at these temperatures, 76(10)% and 62(10)%, respectively, of the most stable conformer i.e. the one having a hydrogen atom eclipsing the double bond, were found. The conformer with the chlorine atom eclipsing the CC bond was also present. However, from the experimental data it was not possible to establish conclusive evidence for the conformer with an eclipsed CH 3 group. Molecular mechanics (MM) calculations and ab initio calculations using a 4-21 basis set were carried out with complete geometry optimization, and calculated parameters from each of the methods were used in combination with the ED data. Such calculations indicated the existence of all three conformers mentioned above. Least-squares analysis including constraints from the ab initio calculation gave as a result the following molecular structure ( r a distances and ??? α angles) for the predominant conformer: r (CC) = 1.337(6) A, r (=CC) = 1.503(4) A, r (CCH 3 ) = 1.522 A, R (CCl) = 1.813(4) A, r (CH)> = 1.089(18) A, ???CCC = 122.9(2.1)°, ???CCC = 112.6(2.2)°, ???CCCl = 109.9(0.2)°, ???ClCCH 3 = 109.3°. = 121.9° and = 110.0(1.3)°. The torsional angles were then τ(CCCCl> = −119.4° and τ(CCCCH 3 ) = 120.3(2.1)°. Error limits are 2σ (σ includes estimates of systematic errors and correlations), parameters without quoted uncertainties are dependent or were constrained relative to another parameter. Combining the ED data with MM results yielded parameters consistent with those given above.

3-Bromo-2-methyl-1-propene: molecular structure and conformation in the gas phase as determined by electron diffraction and molecular mechanics calculation

Abstract A gas phase electron diffraction study of 3-bromo-2-methyl-1-propene shows that there is predominantly a gauche conformer present. Data recorded at 20 and 180°C show 4(8) and 5(4)% respectively of a second confomer with a planar heavy atom skeleton. The gauche structural results in terms of r a distances and ∠ α angles at 20°C were found to be: r (CC) = 1.331(9) A, r (CCH 2 Br) = 1.484(6) A, r (CCH 3 ) — r (CCH 2 Br) = 0.017 A, (assumed), r (CBr) = 1.965(6) A, ∠CCCH 2 Br = 121.5(0.7)°, ∠CCCH 2 Br — ∠CCCH 3 = 0.7° (constraint from molecular mechanics calculation), ∠CCBr = 112.2(0.5)°, torsional angle = 112.5(2.2)°. Uncertainties are given as 2σ, where σ includes uncertainties due to correlation among observations, electron wavelength and other parameters used in the data reduction. The results obtained from the 180°C data agree very well with those given above. The molecular mechanics calculations yield information consistent with the experimental results.