H. Priebe

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.

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.

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.

Deiodination of iodinated aromatic compounds with electrospray ionization mass spectrometry

RATIONALE Dehalogenation of iodinated X-ray contrast media (ICM) has been reported using electrochemical and bioelectrochemical systems. Correspondingly, dehalogenation of aromatic halogens has also been reported in mass spectrometry (MS) using different ionization techniques like chemical ionization (CI), thermospray, fast-atom bombardment (FAB) and FAB-liquid secondary ionization mass spectrometry (LSIMS). The aim of the present work was to study deiodination of iodinated aromatic compounds in MS with electrospray ionization (ESI). METHODS The iodinated aromatic compounds were characterized by liquid chromatography/tandem mass spectrometry (LC/MS/MS) using a quadrupole time-of-flight (QTof)-micro MS instrument and ESI in both positive and negative ion mode. The effect of mobile phase additives like formic acid, acetic acid, trifluoroacetic acid, ammonium formate and ammonium acetate on the negative and positive ESI mass spectra of the iodinated aromatic compounds was studied. RESULTS Formic acid and ammonium formate induced deiodination of the iodinated aromatic compounds with ESI-MS. Neither acetic acid, trifluoroacetic acid nor ammonium acetate induced the deiodination reaction. The effect was most pronounced with negative ESI where the HI product of the deiodination reaction easily adhered to the aromatic compounds giving rise to HI adducts in the mass spectra. The deiodination reaction was shown to take place in the ESI capillary, since the extent of the reaction was largely dependent on the capillary voltage. The calculated heat of reaction for deiodination of the iodinated aromatic compounds was significantly exothermic for formic acid. This was not the case for acetic acid and trifluoroacetic acid. CONCLUSIONS Care should be taken when using formic acid as a mobile phase additive in LC/MS analyses of iodinated aromatic compounds, since the interpretation of the mass spectra might be influenced by potential dehalogenation reactions taking place in the ESI capillary.

The vibrational spectrum of propyne-3d1

Abstract The i.r. spectra of propyne-3 d 1 , in the vapour phase and as a crystalline solid at 90 K were recorded in the region 5000-200 cm −1 . Raman spectra, including semiquantitative polarization data, of the vapour phase, of the neat liquid and of the solid at 90 K were obtained. The fundamental frequencies were assigned in good agreement with the result from a normal coordinate calculation.

The conformation and vibrational spectra of 2-ethynyl-1,3-butadiene (3-methylene-4-pentene-yne)

Abstract A sample of 2-ethynyl-1,3-butadiene was synthesized by a thermal rearrangement of 1,2-hexadiene-5-yne at ca. 770 K. Infrared spectra were recorded of the vapour, the liquid and of the amorphous and crystalline solids at 90 K in the region 4000-50 cm −1 . Raman spectra were obtained of the cooled liquid, including semiquantitative polarization measurements, and of the crystalline solid at 90 K. The spectral data indicate that 2-ethynyl-1,3-butadiene exists as the s-trans conformer in the various states of aggregation but the possibility of small amounts of a second conformer cannot be excluded.