Solvent dependence of nuclear spin-spin coupling constants

. 69, 1039 (1991). The 'H nuclear magnetic resonance spectra of 2-cyanobenzaldehyde (2CNB) and 3-cyanobenzaldehyde (3CNB) in CS2/C6D12 and acetone-d6 solutions at 300 K yield precise stereospecific long-range proton-proton coupling constants. These are used to establish the conformational population of the o-cis and o-trans conformers of these relatively polar molecules. For example, the fractional o-cis population of 2CNB changes from 0.12(4) in CS2/C6DI2 to 0.46(6) in acetone-d6, whereas that of 3CNB is 0.48(2) in both solvents. Extrapolation to the vapor phase, using a dielectric model, implies a negligible concentration of the o-cis conformer of 2CNB and a roughly 50% abundance of each conformer of 3CNB. Computations at various levels of molecular orbital theory provide estimates of the rotational bamer of the aldehyde moiety and confirm the planar structure of each conformer. The geometries of three conformers are given as obtained from the 6-3 IG MO basis and may be useful to molecular spectroscopists. Theoretical and experimental dipole moments are interpolated to yield estimates of their magnitudes for the four planar conformers. Somewhat less precise 'H nrnr spectral parameters (than for the above solutions) are also obtained for dilute solutions in benzene-d6 at 300 K. ' The conformational distributions based on these parameters are compared with their only other measurement, based on dipolar moments in benzene at 298 K. Good agreement between the results of the two methods is found for 3CNB but not for 2CNB. It is suggested that specific interactions occur between benzene solvent and solute molecules, particularly for 3CNB, for which these interactions stabilize the conformer having a low dipole moment. Remarkable changes in the intraring proton-proton coupling constants occur in going from CS2/C6DI2 to acetone-d6 solution.Key words: 2-and 3-cyanobenzaldehyde (2CNB and 3CNB): 'H NMR, conformations, long-range spin-spin coupling constants, MO computations.TED SCHAEFER, KERRY J. COX et RUDY SEBASTIAN. Can. J. Chem. 69, 1039 (1991). Les spectres de resonance magnttique nuclCaire du 'H du 2-cyanobenzaldChyde (2CNB) et du 3-cyanobenzaldthyde (3CNB), mesurtes B 300 K, dans des solutions dans le CS2/C6DI2 et dans I'acCtone-d6, fournissent des constantes de couplage proton-proton 2 1 longue distance trks prCcise. On les a utiliskes pour dCterminer les populations conformationnelles des conformkres o-cis et o-trans de ces molCcules relativement polaires. Par exemple, la fraction de la population o-cis du 2CNB varie de 0,12(4) dans le CS21C6D12 i 0,46(6) dans l'acttone-d4; par ailleurs, celle du 3CNB est Cgale h 0,48(2) dans les deux solvants. Une extrapolation jusqu'h la phase vapeur, utilisant un modkle ditlectrique, suggkre que la concentration du conformkre o-cis du 2CNB est alors ntgligeable, mais que celle du 3CNB est approximativement Cgale a 50%. Des calculs i divers niveaux de la thCorie des orbitales moltculaires fournissent des tvaluations de la barrikre 1 la rotation de la portion aldkhydique et con...

Section: Ii) the Intraring "J(hh)mentioning

confidence: 98%

The conformational distribution of 2-cyanobenzaldehyde and 3-cyanobenzaldehyde in solution and in the vapor

Schaefer¹,

Cox²,

Sebastian³

1991

“…Only more recently has the solvent dependence of coupling constants been recognized and investigated. Several review articles have been published (1)(2)(3), the most extensive compilation being that by Smith (4). However, there seem to be no studies on coordination complexes with the exception of a small amount of data (published subsequent to the completion of our studies) on lJptVp in cis-[ P~C I~( P B U~~)~] and tran~-[PtI2(PBu"3)~] (5).…”

Section: Introductionmentioning

confidence: 99%

Solvent, concentration, and temperature dependance of the platinum–phosphorus nuclear magnetic résonance coupling constants in cis- and trans-[PtCl₂(PBu₃ⁿ)₂]

Dixon¹,

Fakley²,

Pidcock³

1976

. Can. J. Chem. 54, 2733Chem. 54, (1976. Studies of the effects of solvent, concentration and temperature on the 31P nmr parameters of cis-and trans-[PtC12(PBun3)2] are reported. The coupling constants, lJFt-,, are markedly dependent upon all three variables but, in contrast to previous studies on non-coordination compounds, very little corresponding variation in chemical shifts is found. Results are discussed in terms of a solvent cavity model using reaction field, Stark and Macrae effects but it is concluded that a more specific model involving competitive solvation at platinum gives a better interpretation of the data. KEITH R. DIXON, MARTIN FAKLEY et ALAN PIDCOCK. Can. J. Chem. 54, 2733 (1976). Une Ctude sur les effets du solvant, de la concentration et de la temperature sur les parametres du rmn 31P du [PtC12(PBun3)2] cis et trans est rappcrtee. Les constantes de couplage 'JPt-, dependent d'une f a~o n marquee des trois variables mais contrairernent B des Ctudes prkddentes sur des composCs de non-coordination, on ne trouve qu'une trks petite correspondance dans la variation des dCplacements chimiques. Les rCsultats sont discutks en terme d'un modele de cage de solvant utilisant les effets de champs de rhction et les effets de Stark et ~Macrae toutefois, on en conclue qu'un modele plus spkifique impliquant une solvatation competitive du platine donne une meilleure interprktation des resultats.[Traduit par le journal]

“…"J('~c, 1 9~) in 1 'J increases algebraically in going from CS2 to acetone-d6 solution, as expected from an electrostatic model (31). In contrast, the positive 2~(~, F) decreases as polarity of the solvent increases.…”

Section: 'J(h Cf) and Proximate Coupling Mechanismsmentioning

confidence: 54%

¹H, ¹⁹F, and ¹³C nuclear magnetic resonance approaches to the barrier to rotation about the Csp²—Csp³ bond in 1,1,1 -trifluoro-2-phenylethane. Proximate coupling constants and molecular orbital computations

Schaefer¹,

Hazendonk²,

McKinnon³

1995

Abstract:The 'H, I9F and 1 3~( ' H } nuclear magnetic resonance spectra of 1,l ,I-trifluoro-2-phenylethane, 1, in CS2-C,D12, acetone-d6, and benzene-d, solutions, on analysis, yield long-range coupling constants from which are derived the apparent twofold barriers to rotation about the C S~' -C S~~ bonds. The twofold barrier is 9.0(2) kJImol, independent of solvent, 4.0 Idlmol larger than that of ethylbenzene, also independent of solvent. The theoretical barrier heights for the free molecules at the post-Hartree-Fock level of molecular orbital theory (frozen-core MP216-31G*) also differ by 4 kJImol, but are about I kJ1mol higher than the experimental estimates. The perpendicular conformer is the most stable for both molecules. Comparisons are made with the benzyl halides, in which the internal barriers are remarkably sensitive to solvent. A spin-spin coupling constant over five formal bonds, 5~(~, F), involving the ortho protons in 1, is +0.74(2) Hz and is discussed in some detail in terms of its dependence on internuclear distances (possible through-space interactions). The solvent perturbations of 3~(~, F) and of 'J(C, F) are of opposite sign. Other long-range coupling constants or their absence are also pointed out. For example, those between "F and 13c nuclei or protons at the meta position are effectively zero; at the para position they are significant.Key words: 1.1.1-trifluoro-2-phenylethane; 'H, "F, and I3c NMR; long-range spin-spin coupling constants; through-space 'H, "F spin-spin coupling constants; internal rotational potential; molecular orbital computations of internal potential.RCsum6 : L'analyse des spectres de rksonance magnCtique nucltaire du 'H, du "F et du ' 3~(~] du 1,1,1-trifluoro-2-phtnylCthane, 1, en solution dans du CS,-C6D,,, dans de l'actton6-d, ou du benzkne-d6, permet d'obtenir les constantes de couplage B longue distance B partir desquelles on peut deriver les barrikres binaires apparentes h la rotation autour des liaisons ~s~' -C s~~. La barrikre binaire est de 9,0(2) kJImol, elle est indkpendante du solvant et elle est 4,O kJ1mol plus ClevCe que celle de I'tthylbenz&ne qui est aussi indtpendante du solvant. Les hauteurs des barrikres thCoriques des molCcules libres, CvaluCes par la thCorie des orbitales molCculaires B un niveau post-Hartree-Fock (MP216-3 lG* B rCfCrence fixCe), diffkrent aussi par 4,O kJ1mol; elles sont toutefois environ 1 kJ1mol plus ClevCes que les Cvaluations expCrimentales. Dans les deux molCcules, le conformkre perpendiculaire est le plus stable. On a fait des comparaisons avec les halogknures de benzyle dans lesquels les barrikres internes sont particulikrement sensibles au solvant. La constante de couplage spinspin B travers cinq liaisons formelles, 5~(~, F), impliquant les protons ortho du composC 1 est Cgale B +0,74(2) Hz; on en discute en detail en fonction de sa dtpendance sur les distances internucltaires (interactions possibles