A study of lone-pair interactions of tetracyclic diamines by ultraviolet photoelectron spectroscopy

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DIETMAR CHMIELEWSKI, NICK HENRY WERSTIUK, and T~MOTHY A. WILDMAN. Can. J. Chem. 71, 1741 (1993). The conformational preferences and barriers to internal rotation about the S--C(phenyl) bond have been investigated for thioanisole and its 2-fluoro, 2,6-and 3,5-difluoro, and 2,3,5,6-tetrafluoro derivatives. Measurements of long-range spin-spin couplings between the methyl carbon and the para ring proton indicate that the 2-fluoro and 3,5-difluoro compounds prefer conformations with all heavy atoms coplanar. The 2,6-difluoro and 2,3,5,6-tetrafluoro compounds prefer conformations in which the methyl carbon lies in or near the plane perpendicular to the aromatic ring. Semiempirical molecular orbital calculations with the MNDO method indicate that all of the molecules prefer perpendicular conformations while similar calculations with AM1 indicate that all prefer planar conformations. Apparently the conformational behaviour can be quite sensitive to subtle changes in intramolecular interactions, which may indicate improvements to these semiempirical methods. The NMR results have been used to derive an internally consistent set of rotational potentials. Synthetic photoelectron spectra derived from these potentials and the AM I orbital energies are in good agreement with the experimental spectra.DIETMAR CHMIELEWSKI, NICK HENRY WERSTIUK et T~MOTHY A. WILDMAN. Can. J. Chem. 71, 1741 (1993. On a examine les preferences conformationnelles et les banikres 5 la rotation interne autour de la liaison S-C(phCny1e) dans le thioanisole et ses derives 2-fluoro-, 2,6-et 3,5-difluoro-et 2,3,5,6-tktrafluoro-. Des mesures des couplages spinspin longue distance entre le carbone du mkthyle et le proton en para du cycle indiquent que, dans les conformations prefkrees des composCs 2-fluoro-et 33-difluoro-, les atomes lourds sont coplanaires. Dans les conformations privilCgiCes des compos&s 2,6-difluoro-et 2,3,4,6-tktrafluoro-le carbone du groupe mkthyle se trouve dans le, ou prks du, plan perpendiculaire au noyau aromatique. Des calculs semiempiriques d'orbitales mol&culaires avec la mCthode MNDO indiquent que toutes les molCcules prkferent des conformations perpendiculaires alors que des calculs semblables 5 l'aide de la mkthode AM1 indiquent que chacun prefkre les conformations planaires. I1 semble que le comportement conformationnel peut Ctre trks sensible a des changements subtils dans les interactions intramolCculaires; ceci peut indiquer des mkthodes pour ameliorer les methodes semiempiriques. On a utilisC les rtsultats de la RMN pour developper un ensemble autocoherent de potentiels rotationnels. Les spectres photoClectroniques de synthkse derives de ces potentiels et les energies orbitalaires obtenues a l'aide de la mkthode AM1 sont en bon accord avec les spectres experimentaux.[Traduit par la redaction] Introduction The important role of molecular conformation in the determination of chemical reactivity and molecular recognition is abundantly clear. In addition to experimental probes, quantum mechanical models of molecular electroni...

“…Photoelectron spectra were recorded on a noncommercial instrument described previously (10). Spectra were obtained by signal averaging of 20-30 scans.…”

Section: Experimental and Computational Methodsmentioning

confidence: 99%

Conformational preferences and barriers to internal rotation in fluorothioanisoles from long-range spin–spin couplings, photoelectron spectroscopy, and semiempirical molecular orbital calculations

Chmielewski¹,

Wildman²

1993

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“…The pe spectra of l a and l b were obtained on a non-commercia1 instrument (21). The spectra were obtained by signal averaging 20-30 scans and argon was used as a calibrant.…”

Section: Methodsmentioning

confidence: 99%

Computational and ultraviolet photoelectron spectroscopic evidence that (Z)-2-methyl-1,3-pentadiene prefers twisted s-cis conformers in the gas phase

Timmins¹,

Ma²,

Wildman³

1992

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A redetermination of the ultraviolet photoelectron (pe) spectrum of (Z)-2-methyl-l,3-pentadiene has led to a correction of the published spectrum. By studying (a-2-methyl-1,3,-pentadiene (la) and (E)-2-methyl-1,3-pentadiene (lb) with MMX, MNDO, A M l , and a b initio MO computational methods and pe spectroscopy, we have shown that a combination of these methods provides useful insights on the conformational behaviour of methyl-substituted 1,3-dienes in the gas phase. Synthetic pe spectra, derived from the computed potential energy surfaces and angle-dependent orbital energies, are in good agreement with experiment. Thus, the E isomer prefers the s-trans conformer but the Z isomer prefers twisted s-cis conformations in the gas phase.NICK HENRY WERSTIUK, GEORGE TIMMINS, JIANGONG MA et TIMOTHY A. WILDMAN. Can. J. Chem. 70, 1971 (1992. Introduction A comprehensive study of the effect of methyl and ethyl substituents on the conformations of conjugated dienes by ultraviolet photoelectron spectroscopy (UPS) and by empirical (MMX), semiempirical (AM 1, MNDO), and ab initio MO calculations was initiated recently (1, 2). This work was undertaken for several reasons. The primary goal was to establish whether or not AM1 (3) is useful for carrying out conformational analyses and for detailing conjugative and homoconjugative interactions in dienes. In our view, this research is timely because there is renewed interest (4-8) in the conformational analysis of dienes, a subject which occupied the attention of experimentalists and theoreticians for decades.' Our plan was to study a series of alkyl-substituted 1,3-dienes computationally to calculate heats of formation or total energies as a function of the dihedral (twist) angle between the double bonds in order to identify the stable conformations and to calculate the a_, a+ splittings of planar (s-cis and s-trans) and twisted conformations. By comparing the latter values with the splittings obtained experimen-

“…The He(1) pe spectra of E-3, 2-3, E-4, and 2-4 were obtained on a pe spectrometer (22) operated in the conventional mode, and are averages of 50-1 00 scans.…”

Section: Ultraviolet Photoelectron Spectroscopymentioning

confidence: 99%

Conformational analysis and structure elucidation of 2,3-dimethyl- and 2,4-dimethyl-2,4-hexadienes by AM1 and He(I) ultraviolet photoelectron spectroscopy

Clark¹,

Leigh²

1990

. Can. J. Chem. 68, 2078 (1990). A study of (E)-2,3-dimethyl-2,4-hexadiene (E-3) and (2)-2,3-dimethyl-2,4-hexadiene (2-3) by the semiempirical computational method AM1 and He(1) ultraviolet photoelectron spectroscopy (UPS) has established that this combination of AMl/LIPS will be generally useful for detailing conformational and electronic properties of alkyl-substituted 1,3-dienes. This method was used to elucidate the structures of the stereoisomeric 2,4-dimethyl-2,4-hexadienes obtained from the photolysis of 1,3,3,4-tetramethylcyclobutene (2).