A. Gatial

The vibrational and NMR spectra, conformations and ab initio calculations of aminomethylene, propanedinitrile and itsN-methyl derivatives

The IR and Raman spectra of aminomethylene propanedinitrile (AM) [H2N-CH=C(CN)2], (methylamino)methylene propanedinitrile (MAM) [CH3NH-CH=C(CN)2] and (dimethylamino)methylene propanedinitrile (DMAM) [(CH3)2N-CH=C(CN)2] as solids and solutes in various solvents have been recorded in the region 4000-50 cm−1. AM and DMAM can exist only as one conformer. From the vibrational and NMR spectra of MAM in solutions, the existence of two conformers with the methyl group orientedanti andsyn toward the double C=C bond were confirmed. The enthalpy difference δH0 between the conformers was measured to be 3.7±1.4 kJ mol−1 from the IR spectra in acetonitrile solution and 3.4±1.1 kJ mol−1 from the NMR spectra in DMSO solution. Semiempirical (AM1, PM3, MNDO, MINDO3) and ab initio SCF calculations using a DZP basis set were carried out for all three compounds. The calculations support the existence of two conformersanti andsyn for MAM, withanti being 7.8 kJ mol−1 more stable thansyn from ab initio and 8.6, 13.4, 11.6, and 10.8 kJ mor−1 from AM1, PM3, MNDO, and MINDO3 calculations, respectively. Finally, complete assignments of the vibrational spectra for all three compounds were made with the aid of normal coordinate calculations employing scaled ab initio force constants. The same scale factors were optimized on the experimental frequencies of all three compounds, and a very good agreement between calculated and experimental frequencies was achieved.

The vibrational spectra including matrix isolation and conformations of 1-chloro-1-fluorocyclobutane and 1-chloro-1,2,2-trifluorocyclobutane

Abstract The IR and Raman spectra of 1-chloro-1-fluorocyclobutane (CFCB) and 1-chloro-1,2,2-trifluorocyclobutane (CFCB) were recorded in various phases and at different temperatures, and included IR matrix spectra in argon and nitrogen matrices using the hot nozzle method. Both compounds have two conformers in the vapour and liquid states. CTFCB crystallized as one distinct conformer, while two conformers were present in all solid state spectra of CTFCB. The argon matrix spectrum of CFCB contained only one conformer at 13 K, suggesting a barrier lower than 5 kJ mol−1 between the conformers. In both the argon and nitrogen matrix spectra of CTFCB two conformers were observed, although the concentrations of the less stable conformer were lower than in the vapour. This suggests a partial conversion to the stable conformer during deposition and a barrier slightly higher than 6 kJ mol−1. The enthalpy difference ΔH° between the conformers was 2.2 kJ mol−1 for CFCB in the liquid and 2.7 and 2.9 kJ mol−1 in the vapour and liquid, respectively, for CTFCB. Ab initio calculations were carried out for both compounds using the 3-21G* and 6-31G* basis sets. For CFCB, both calculations gave the conformer with the fluorine atom equatorial as the more stable. For CTFCB, the smaller basis set also gave the conformer with equatorial fluorine as the more stable while the opposite was predicted with the larger basis set. Fairly complete assignments for both conformers of both compounds were made with the aid of normal coordinate calculations employing scaled ab initio force constants. Agreement was excellent between calculation and experiment for the case in which the more stable conformer was that with fluorine equatorial but impossible to achieve with fluorine axial.

Vibrational spectra including matrix isolation and conformations of 1,1,2-trichloro-2,3,3-trifluorocyclobutane

Abstract The IR and Raman spectra of 1,1,2-trichloro-2,3,3-trifluorocyclobutane have been reinvestigated, including matrix isolation IR spectroscopy in argon and nitrogen matrixes, using the hot nozzle technique. New Raman spectra of the vapour, liquid, plastic and crystalline phases were recorded, and IR high pressure spectra of the liquid and solid phases were obtained. The compound has two conformers of nearly equal stability, with an enthalpy difference (Δ H 0 ) equal to 0.0±0.9 in the vapour, 0.9±0.7 in the liquid and 0.4±0.3 kJ mol −1 in the plastic phase, while the values +0.4 (argon) and −0.2 (nitrogen) kJ mol −1 for the vapour were obtained from the unannealed matrix spectra. The argon and nitrogen matrixes favoured the more stable and the less stable conformer, respectively. A Δ H 0 value of −0.5±0.2 kJ mol −1 in the nitrogen matrix was obtained by measuring the conformational equilibrium between 30 and 33 K. The barrier to inversion was found to be about 9 kJ mol −1 in the nitrogen matrix. The Δ G 0 values for conformational change in the two matrixes were calculated from the spectra to differ by 0.8 kJ mol −1 . A plastic phase containing both conformers in thermodynamic equilibrium was present between 194 and 240 K under atmospheric pressure, whereas an anisotropic phase existed below 194K. A considerable hysteresis was obtained both for the melting and phase transitions. The plastic and anisotropic crystals were also observed under pressure and studied in a polarization microscope. The more stable conformer was present both in the low temperature and in the high pressure crystals. Ab initio force constants were calculated for both conformers and an energy difference between the two of 0.79 kJ mol −1 was obtained using the 3–21 G * basis set. The scaled force constants gave wavenumbers in good agreement with the experimental values, and complete assignments of the fundamentals are presented for both conformers. From the force constant calculation it is clear that the more stable conformer is that in which the fluorine atom on the asymmetric carbon is in the equatorial position.

The conformation and vibrational spectra including matrix isolation of 1,1,2-trichlorotrifluoroethane

Abstract The IR spectra of 1,1,2-trichlorotrifluoroethane (FREON 113) have been observed in the vapour phase, in the liquid, in solution, at low temperatures and at high pressures between 4000 and 50 cm−1. Additional IR spectra of the compound isolated in argon and nitrogen matrices at ca. 12 K have been recorded in the 1300–200 cm−1 range. Raman spectra of the vapour, neat liquid and solid at various temperatures have been obtained. When compressed in a diamond anvil cell at ambient temperature, the compound forms a plastic phase at ca. 15 kbar, containing the same two conformers, C1 and Cs, as in the vapour and the liquid. At a pressure exceeding 125 kbar an anisotropic crystal is formed containing the C1 conformer. All the low temperature solid phases studied, appear to be plastic, and both conformers are present. No crystal containing one conformer has ever been achieved at low temperature, in spite of numerous attempts with different cooling techniques down to 15 K. Variation of the nozzle temperature of the matrix isolation unit between ambient temperature and 900 K gives rise to different conformer ratios in the matrices. By comparing band intensities at the various nozzle temperatures, the enthalpy difference of the two conformers is estimated to be 1.06 ± 0.11 kJ mol−1 with C1 as the more stable. A tentative assignment of IR and Raman bands to the C1 and Cs fundamentals is carried out, supported by a normal coordinate calculation involving related molecules.

The vibrational spectra and conformations of polyhalogenated cyclobutanes

Abstract The vibrational spectra of 1-chloro-2,2,3,3-tetrafluoro- and 1-cyano-2,2,3,3-tetrafluorocyclobutane reveal that these compounds have a conformational equilibrium strongly shifted towards equatorial Cl and CN substituents, although to a smaller extent than for the chloro and cyanocyclobutane. When a Cl and an F atom are attached to the same carbon, they will compete for the preferred equatorial position, giving relatively small enthalpy differences between the conformers. In 1-chloro-1-fluoro-, 1-chloro-1,2,2-trifluoro- and 1,1,2-trichloro-2-3,3-trifluorocyclobutane the enthalpy differences are 2.2, 2.9 and 0.9 kJ mol −1 in the liquid, respectively. In 1-chloro-1,2,2-trifluro and 1,1,2-trichloro-2,3,3-trifluorocyclobutane the ring puckering barrier was ca. 6–7 kJ mol −1 , for the remaining compounds it was smaller than 5 kJ mol −1 .

IR Matrix isolation studies of the conformations of 1,3-dichloro-2,2-dimethylpropane and 2-(chloromethyl)-2-methyl-1,3-dichloropropane

Abstract The IR spectra of the title compounds matrix isolated in argon and nitrogen at 14 K, were recorded with the hot nozzle method at temperatures of 300, 450, 700 and 900 K. In 1,3-Dichloro-2,2-dimethylpropane a third conformer (AA) with C 2v symmetry was detected in addition to those with C 2 (GG) and C 1 (AG) symmetries observed previously. The enthalpy differences were measured as ΔH o (AG-GG) equal to 4.0 and ΔH o (AA-GG) equal to 7.5 kJ mol −1 . In 2-(Chloromethyl)-2-methyl-1,3-dichloropropane the spectra revealed the three conformers with symmetries C S , C 1 and C 3 determined earlier, and the enthalpy differences were measured to be ΔH o (C 1 −C S )=1.7 and ΔH o (C 3 —C S )=3.9 kJ mol −1 .

Vibrational spectra and conformations of 1,3-dibromo-2,2-dimethylpropane (DBDMP) and 2,2-di(fluoromethyl)-1,3-difluoropropane (TFN)

Abstract Two of the four possible conformers for DBDMP have been observed in the liquid phase. At low temperature DBDMP crystallizes in the GG conformation (C 2 ), and at high pressure in the AG conformation (C 1 ). At least three of the possible six conformers have been observed for TFN. Two are in equilibrium at room temperature, and a third appears only in the matrix spectra deposited from a hot sample.

Dimethyl 2-(amino-methyl-ene)malonate.

In the title compound, C6H9NO4, which is an example of a push–pull alkene, N—H⋯O interactions stabilize the crystal structure.

Ab initio study of 4-azidobutine

Abstract The geometry optimization of all five distinct conformations of 4-azidobutine (with labelling GG, GA, GG′, AA and AG) has been carried out on the ab initio TZ + P SCF level. MP2 calculations have been performed for all molecules at the SCF geometry. Our theoretical and experimental results indicate that the conformers can be divided into two groups: the first group with low-energy conformers (GG, AG and AA) and the second one with high-energy conformers (GG′and GA). A decrease of the possible interation of π-systems of the CC bond with the azide group in comparison with the allylazide molecule was observed.

The IR, Raman and NMR spectra and conformations of cyclohexylallene

Abstract Three conformers of cyclohexylallene, designated I, II and III, were observed in the IR and Raman spectra. Metastable crystals formed by annealing an amorphous solid from 80 K to 140 K, and crystals formed under a pressure of 30 kbar at ambient temperature, contained conformer II. The stable crystals obtained after heating the metastable crystals to 195 K contained conformer I. The conformer III was observed as a minor component in a nitrogen matrix heated to 900 K before deposition. Negligible intensity variations between the bands of conformer I and II were observed with nozzle temperatures 300–900 K in the argon and nitrogen matrices, indicating a ΔHo(I-II) below 0.4 kJ mol−1 while ΔHo(III-II) was equal to 6.5 kJ mol−1 and a barrier higher than ca. 11 kJ mol−1 was observed between the latter. Variable temperature measurements in Raman indicate ΔHo(I-II) ≈ 0.9 kJ mol−1 in the liquid. The 13C NMR spectra show that in deuteromethanol solution the equatorial conformer dominates (≈ 2% a at 193 K, ΔGo(a–e) ≈ 6.4 kJ mol−1). Analysing the temperature dependence of the vicinal H-H coupling across the cyclohexane - allene carbon bond on the basis of a guache ↔ anti equilibrium it is found 40% anti and 60% gauche in methanol solution at ambient temperature. We conclude that the abundant conformers II and I being respectively the anti and the gauche conformer, while III is the anti axial conformer.