Henry V. Secor

Capillary gas chromatographic separation of enantiomers with stable dipentyl-α-, β- and γ-cyclodextrin-derivatized stationary phases

Abstract Liquid and amorphous solid pentyl derivatives of α-, β- and γ-cyclodextrin were evaluated as gas chromatographic (GC) chiral stationary phase coatings on fused-silica capillaries. The factors controlling the physical state of the pentyl-substituted cycldodextrin derivatives were considered. Sixty-five racemic solutes were resolved, including alcohols, amines, lactones, epoxides, amino alcohols, esters, ethers, amino acids, haloalkanes, sugars and alkaloids. The pentyl-substituted α-, β- and γ-cyclodextrin stationary phases showed pronounced selectivity differences, apparently based on the size, shape and functionality of the analytes. Inclusion complexation may more readily occur at high temperatures in gas than in liquid chromatography (LC). Enantiomeric separations can be accomplished by this GC method that cannot be done by related cyclodextrin LC techniques.

Conformational changes upon S1←S0 excitation in 4-dimethylaminobenzonitrile and some of its chemical analogs

One‐color time‐of‐flight mass spectra (mass resolved excitation spectra) for jet‐cooled 4‐dimethylaminobenzonitrile (4‐DMABN) and some of its chemical analogs, dimethylaniline (DMA), 3‐dimethylaminobenzonitrile (3‐DMABN), N, N‐dimethyl‐4‐(trifluoromethyl)aniline (4‐CF3‐DMA), and 4‐(d6‐dimethylamino)benzonitrile (4‐d6‐DMABN), are presented and analyzed. Near the origin of the S1←S0 transition the low frequency modes can be assigned to motions of the dimethylamino group for this series of molecules. The inversion motion of the dimethylamino group and the dimethylamino group torsion about the Cipso–N bond (the twist coordinate) in S1 give rise to the most prominent peaks in this spectrum. The potential parameters for the twist coordinate of 4‐DMABN and DMA are quite similar in S1: B=0.546, V2=175, and V4=525 cm−1 for 4‐DMABN and B=0.546, V2=175, and V4=515 cm−1 for DMA. The V2 and V4 terms are slightly larger for 3‐DMABN and 4‐CF3‐DMA. The inversion motion is also similar for these molecules but is more anha...

Gas phase versus solution chemistry: on the reversal of regiochemistry of methylation of sp2- and sp3-nitrogens

Abstract The nicotine analogue 3-( N , N -dimethylaminomethyl)pyridine 2 , which reacts with methyl iodide in acetonitrile exclusively on the sp 3 -nitrogen, methylates exclusively on the pyridine nitrogen with trimethyloxonium in the gas phase. Calculations at the RHF/6-31G**//6-31G** level suggest that there is an interaction between the side chain nitrogen and hydrogens on the aromatic ring. This interaction affects both the conformation of 2 as well as the relative basicity of its two nitrogens. Calculations also indicate that, in the gas phase, the pyridine methylated product (no counterion) is more stable than the pyrrolidine methylated product.

Separation of homologous and isomeric alkaloids related to nicotine on a β-cyclodextrin-bonded phase

Abstract Nicotine and thirteen structurally related alkaloids were separated with a β-cyclodextrin-bonded phase column. The retention of homologous compounds tends to increase as the size of the homolog increases. Structural isomers in which the pyridine nitrogen is in the 4-position are always retained the longest. Both retention and efficiency are very pH dependent. Conversely, selectivity does not change significantly with pH.

Assessment of isolated electronic effects on conformation. NMR analysis of nicotine and related compounds and ab initio studies of model compounds

Abstract The influence of electronegative substituents on the N ′-methyl group of nicotine upon the conformation of the pyrrolidine ring has been evaluated by the exact analysis of the high field 1 H NMR spectra of nicotine ( 1 ), N ′-ethylnornicotine ( 2 ), N ′-(2,2-difluoroethyl)-nornicotine ( 3 ) and N ′-(2,2,2-trifluoroethyl) nornicotine ( 4 ). The vicinal coupling constants for the pyrrolidine ring of 1–4 remain nearly constant, suggesting that as the electronegativity of the N ′-methyl substituent increases, only very small changes are seen for the C 3′ —C 4′ —C 5′ —N′ and the C 2′ —C 3′ —C 4′ —C 5′ dihedral angles. Substitution on the N ′-methyl group appears to have little effect on the orientation of the pyridyl ring with respect to the pyrrolidine ring. Ab initio calculations have been performed on the analogous 2-substituted diethylamines (diethylamine, N -ethyl-2-fluoroethylamine, N -ethyl-2,2-difluoroamine, and N -ehtyl-2,2,2-trifluoroethylamine) which constitute substructure models of 1–4 . These calculations confirm the NMR results in that they both indicate little, if any, effects on the rotational barriers and conformational energy profiles as a function of number of fluorine atoms.

Minimum energy conformation of ortho‐xylene in its ground and first excited electronic states

The geometry of ortho‐xylene is studied through supersonic jet cooling and one color mass resolved excitation spectroscopy. By examining the S1←S0 transition origin region of various (d0–d2) methyl deuterated o‐xylene species, the conformation of the two methyl groups in S0 and S1 can be determined. A comparison between the predicted and experimentally observed number and intensity of origin features for the various partially deuterated o‐xylenes shows that two highly symmetric methyl group structures are possible, each structure having C2V point group symmetry—the antiplanar conformation (A1), in which τ1(C2–C1–Cα–Hα)=180°; τ2(C1–C2–Cα’–Hα’)=180°; and the syn, planar conformation (A2) in which τ1=τ2=0°. The experimentally determined structures are consistent with ab initio calculations and microwave studies which predict the anticonformation to be most stable. Spectroscopic data, in conjunction with further ab initio calculations, are used to investigate the ground and excited state potential energy surf...

The radical and organometallic methylation of nicotine and nicotine n-oxide☆

Abstract 2-Methylnicotine is a major product in the reaction of nicotine with methyllithium and methyl radical, in addition to the previously reported 4- and 6-methylnicotines. The reaction of nicotine N-oxide with methylmagnesium bromide furnishes both 2- and 6-methylnicotine.

Conformational analysis through selective isotopic substitution: supersonic jet spectroscopic determination of the minimum energy conformation of o-xylene

Supersonic jet mass resolved excitation spectroscopy is employed to observe the S1â†� S0 electronic transition origin of the individual conformations of [α-2H1]-, [α,α-2H2]- and [α,α′-2H2]-o-xylene and to establish the minimum energy conformation of o-xylene in its ground (S0) and first excited (S1) singlet states. The anti, planar, gear clashed conformer 5[see Tables 1 and 2; i.e., τ(C-2–C–1-Cα–H-α)=τ(C-1–C-2–C-α′–H-α′)= 180°] is found to be the most stable.

Supersonic jet studies of benzylamines. Geometry of their minimum energy conformations

Abstract Supersonic jet mass resolved excitation spectroscopy is employed to demonstrate that the minimum energy conformation of benzylamine and a number of its ortho-unsubstituted (i.e., sterically unhindered) derivatives is the perpendicular form for which τ(C ortho -C ipso -C α -N) = 90°.

Observation and geometry assignment of the conformation of benzyl alcohol in the gas phase

Supersonic molecular jet laser spectroscopy is used to establish the perpendicular conformation of benzyl alcohol and a number of sterically unencumbered derivatives in the gas phase.