J. Phys. Chem. 1994,98, 1213-1216
1213
Orientational Order of Liquid-Crystalline 4-Alkyl-N- (pcyanopbeny1)piperidines by 2Hand 13C NMR Spectroscopies+*' Bashir M. Sheikh-Ali,t C. L. Khetrapal,! and Richard G. WeIss**$ Department of Chemistry, Georgetown University, Washington, D.C. 20057, and Sophisticated Instruments Facility, Indian Institute of Science, Bangalore 560 01 2, India Received: June 22, 1993; In Final Form: November 9, 1993"
The orientational order of nematic 4-alkyl-N-(4-~yanophenyl)piperidines(I) has been determined from 2H and 13C N M R spectra. Molecular-order parameters are derived from the carbon-1 3 chemical shift of the cyano carbon atom in the nematic and the isotropic phases; the sign of the diamagnetic anisotropy is positive. Deuterium quadrupolar splittings from the partially deuterated piperidine ring of I are then related to various C-D bonds.
Introduction
Previously we have reported that a 1/1 (wt/wt) mixture of N-(4-cyanophenyl)-4-n-heptylpiperidine (I, n = 7) and N44cyanophenyl)-4-n-butylpiperidine (I, n = 4),designated 1-47,
I becomes nematic when cooled to below 8 0C.2 Other mixtures of I (n = 4-9) also form monotropic nematic phases, 2b but individual homologues do not except for I (n = 6).3 In order to understand this interesting and rather rare behavior, we have attempted to characterize the orientational order parameters of nematic binary mixtures of I by a combination of I3C and 2H spectroscopies. For the sake of convenience, we have chosen a 1/1 wt/wt mixture of the I (n = 5) and I (n = 7) homologues (1-57) since its nematic clearing point (18.6 "C) and stability in themesophaseare higher than thoseof 1-47. The phase diagrams of the n = 5/n = 7 and n = 4/n = 7 mixtures are qualitatively similar, and the optical birefringent patterns of their nematic phases are indistinguishable. The N M R data, in combination with molecular structures calculated by the MNDO method and optical spectroscopic measurements,2bprovide a detailed picture of molecular packing within the nematic phases of 1-57. The NMR spectra of molecules in liquid-crystalline environments are dominated by the partially averaged components of the second-order tensor properties representing chemical shielding ui,dipolar coupling Dij, and quadrupolar interactions Qi.4 When the magnetic field experienced by a nematic sample induces molecular alignment, so that the orientation of the molecular director is the same on the average for the whole sample, the observed IH dipolar and 2Hquadrupolar splittings and the protondecoupled 13Cline positions in the NMR spectra can be related to orientational order.5
Experimental Part Synthesis of I and identification of the nematic phases formed by their mixtures by differential scanning calorimetry and optical microscopy have been reported elsewhere.2 Partially deuterated 4-pentylpiperidine was obtained by shaking 4-pentylpyridine in
* Author to whom correspondence should be addressed.
Dedicated to Professor C. N. R. Rao on the occasion of his 60th birthday. Georgetown University. $ Indian Institute of Science. e Abstract published in Aduance ACS Abstracts, January 1, 1994. t
2H20/2HC1under 40 psi of IH2 gas for 20 h at room temperature in the presence of platinum (added in catalytic amounts as Pt02).6 Nucleophilic aromatic substitution by the partially deuterated 4-pentylpiperidine on 4-fluorobenzonitrile afforded partially deuterated I (n = 5 ) . 'H NMR (CDCl3, TMS) spectra of the partially deuterated I ( n = 5) and its precursor indicate an unequal number of deuterons at the different positions of the ring. Protondecoupled I3C NMR spectra confirm the presence of deuterium at all positions of the piperidyl ring since its three carbon signals appear as asymmetrical multiplets. A 50/50 (wt/wt) mixture of protiated I (n = 7) and partially deuterated I (n = 5) forms a monotropic nematic phase at 17 OC, within 2O of that of the corresponding nondeuterated binary mixture.2b Partially deuterated and nondeuterated mixtures were used for the deuterium and carbon- 13 NMR studies, respectively; both samples are referred to as 1-57. Individual components were >99% pure by gas chromatographic analysis. 2H NMR spectra of 1-57 in its nematic and isotropic phases were obtained on stationary samples at 46.07 MHz using a quadrupolar echo pulse sequence' with a Bruker AM-300 WM spectrometer, an Aspect 3000 computer, a CP/MAS probe, and an Amplifier Research 200L amplifier. The s / 2 pulse was 2.5 ps. Quadrupolar echoes were recorded with 1 K data points over a 250-KHz spectral width. The second delay in the echo sequence was adjusted to allow data acquisition to start at the echo signal maximum. A 1-s delay between scans was used. To ensure that samples were in the nematic phase during data acquisition, spectra were recorded periodically with a 5-s delay. In some spectra, phasing problems (manifested in the baselines) appeared. Several manipulations, including "left-shifting" of the quadrupole echoes, were attempted, but none improved the quality of the baselines. Proton-decoupled 13CNMR spectra of nematic 1-57 wereobtained from static samples at 75.4 MHz with 50-KHz decoupling power supplied in the CW mode, a 10-ms acquisition time, and a 5-s delay between pulse sequences. Temperature in the sample probe was calibrated using the isotropic-to-nematic phase transition of cooled 1-57 as a standard. Completeoptimization of geometrical parameters of I (n = 5) were calculated using a standard MNDO method.* Results and Discussion
N M R Spectra. A dilute solution-phase proton-decoupled carbon-13 N M R spectrum of I ( n = 5 ) is shown in Figure la. Assignment of the resonances from aromatic carbon atoms is based on their analogy to the resonances from other parasubstituted benzonitriles. The carbon signal of the nitrile group generally appears at 110-120 ppmdownfield fromTMS.g-'l From gatedecoupled 13Cspectra, the 114 and 133 ppm signals in Figure l a can be assigned unambiguously to proton-bearing carbons.
0022-365419412098-1213%04.50/0 , 0 1994 American Chemical Society I
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Sheikh-Ali et al.
1214 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994
anisotropy as shown in eq 1:13
I1
a
200
180
160
140
120 100 PPM
6 , ~ = 61~0+ 2/$'zzAa + ' / 3 (Sxy-Syy)(ax.x-~yy) (1) In this equation, is the isotropic chemical shift value, h a = [crzz- '/2(u= + aW)]is the chemical shielding anisotropy, and a ~ am, , and azz are the diagonal elements of the chemical shielding interaction tensor. Sxx, S w , and SZZare the diagonal elements of the 3 X 3 ordering matrix. S u - Sw for rodlike molecules is
