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8539777 
Journal Article 
Molecular complexes. Part 12. Dimeric toluene, torsional vibrations, dipoles and isomeric complexes in 1H NMR studies of weak arene complexes. Temperature dependence of CH signals 
Stamm, H; Strumm, HO; Loebel, KH; Jäckel, H; Homanns, U; Bethke, S; Schilling, G 
2001 
11 
2089-2095 
English 
The refined (AUS concept, CCl4, external ref.) 1H NMR method provided association constants K and approximate complex shifts IK for stacking complexes of aromatic hydrocarbons D with A when A is 4-nitrobenzaldehyde (1) or is related to 1. D covers the benzene ring of A but torsionally vibrating substituents in A influence topology and K. In the absence of D, vibrations of A can be slowed by low temperatures making A signals go downfield in accord with an increased planarity of A. Vibration of CHO (a dipole) in complexes of 1 with benzene B or toluene T is made non-symmetric by interactions with the quadrupole of D. The large naphthalene (N) hinders vibrations thus enhancing the contact interface by a more planar 1 whose intramolecular deshielding of protons is increased providing small IK values. Aldehyde 1 forms complexes both with T and with its stacking dimer T2 since IK values for 1-T are significantly greater than for 1-B. Complexing with T2 is not found when the molecular dimensions of A allow a dipole-dipole interaction with T as in face-to-face complexes of 1-ethyltheobromine or of 1-chloro-2,4-dinitrobenzene (7). Different protons give slightly different values of K for 7-T and this points to an isomeric edge-on complex with both T and T2 where K for the two edge-on protons (shortest distance to D) is the sum of the binary K and the small K for the the ternary (7 + T2) complex. Compound 7 and 1,3,5-trimethylbenzene show some of the effect described for 1-N. 
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