Long-range proton coupling constants and molecular orbital calculations as indicators of conformational populations of benzene-1,2- and -1,3-dicarbaldehydes

Abstract

. Can. J. Chem. 64, 158 (1986).The 'H nrnr spectra of the benzene-1,2-and -1,3-dicarbaldehydes in carbon tetrachloride, benzene-d6, and acetone-d6 solutions at 300 K are analyzed. The stereospecific long-range couplings over five formal bonds between the sidechain and ring protons show that the 1,2 isomer exists as an 87: 13 mixture of the cis-trans and trans-trans conformers in carbon tetrachloride. These populations are insensitive to solvent. Molecular orbital calculations utilizing extensive geometry optimization procedures imply that the cis-cis form, with proximate C=O bonds, is indeed of negligible significance as assumed in obtaining the populations of the other forms. Further calculations define a pathway of relatively low energy for interconversion of the two abundant forms, in agreement with dynamic nrnr studies. For the 1,3 isomer the long-range couplings provide a check of the conformer populations deduced from dipole moment and "C nmr studies. For example, if the cis-trans form is 70% abundant, as deduced from the dipole moment in benzene solution, then the long-range couplings imply that the population of the cis-cis conformer is insignificant. TED SCHAEFER, GLENN H. PENNER, RUDY SEBASTIAN et CRAIG S. TAKEUCHI. Can. J. Chem. 64, 158 (1986). On analyse les spectres rmn du 'H des benzkne dicarbaldehydes-1,2 et -1,3 qui ont t t t mesurCs a 300 K et dans des solutions dans le tttrachlorure de carbone, le benzkne-d6 et 1'acCtone-d6. Le couplage stCrCospCcifique a longue distance a travers cinq liaisons normales entre les protons du cycle et ceux de la chaine latCrale indique que dans le tktrachlorure de carbone l'isomkre 1,2 existe sous la forme d'un mtlange 87: 13 des conformkres cis-trans et trans-trans. Le solvant n'a aucune influence sur cette rtpartition. Les calculs d'orbitales molCculaires, faisant appel a des procCdCs extensifs d'optimisation gComCtrique, suggkrent que la forme cis-cis ayant des liaisons s'approchant de C=O n'a pratiquement aucune importance dans la dktermination des populations des autres formes. Des calculs plus pousses dkfinissent un chemin rkactionnel de faible Cnergie pour l'interconversion des deux formes abondantes et ceci est en accord avec les Ctudes de la rmn dynamique. Dans le cas de l'isomkre-1,3, les couplages B longue distance constituent une ~Crification des populations des conformkres qui ont CtC dCduites a partir des Ctudes du moment dipolaire et de la rmn du I3C. Par exemple, si la forme cis-trans est a 70%, tel que dCduit t i partir du moment dipolaire en solution dans le benzkne, alors le couplage a longue distance implique que la population de la forme cis-cis n'est pas significative.[Traduit par le journal]

Matrix-Isolation Infrared Spectroscopy of the Rotational Isomers of 1,2-, 1,3-, and 1,4-Benzenedicarboxaldehyde

“…The presence of the two rotamers has been indicated by NMR, infrared, Raman, and dipole moment measurements for 1,4-BDA [1][2][3][4][5][6][7] and by NMR for 1,2-and 1,3-BDA. [8][9][10] Infrared spectral data have been reported also for 1,2-and 1,3-BDA. [11][12][13] However, the definite spectroscopic separation and the band assignments have not been carried out for the rotamers of 1,2-and 1,3-BDA.…”

“…1,2-, 1,3-, and 1,4-BDA are also called phthalaldehyde, isophtalaldehyde, and terephthalaldehyde, respectively. The presence of the two rotamers has been indicated by NMR, infrared, Raman, and dipole moment measurements for 1,4-BDA − and by NMR for 1,2- and 1,3-BDA. − Infrared spectral data have been reported also for 1,2- and 1,3-BDA. − However, the definite spectroscopic separation and the band assignments have not been carried out for the rotamers of 1,2- and 1,3-BDA. Further, the detailed information on the energetic relationships among the rotamers was not available for 1,2- and 1,4-BDA, and infrared spectroscopy has so far not provided any evidence for the existence of the three rotamers for 1,3-BDA.…”

Ground-State, Excited-Singlet-State, and Excited-Triplet-State Energy Levels and Photophysics of the Three Rotational Isomers of Isophthalaldehyde Vapor

“…The evidence for the existence of the three rotamers has not been provided so far for this molecule, although the presence of the two rotamers has been indicated by NMR. [2][3][4] Infrared spectral data also have been reported, [5][6][7] but the infrared spectroscopy has not provided any evidence for the existence of the three rotamers. Further, the detailed analyses of the vapor-phase emission spectrum are not available in the literature.…”