Steric effects on mesomerism. XVII. Some properties of aromatic nitro‐, amino‐, and acylamino compounds with bulky ortho‐substituents

Abstract

Following up earlier work, this paper discusses the influence of 2-methyl-6-tert.buty1 substitution and/or 2,6-di-fert.butyl substitution on certain properties of compounds such as nitrobenzene. aniline, N-methylaniline. N.N-dirneBylaniline. acetanilide, 4-nitro-aniline, and 4-nitro-acetanilide.For , h e derivatives of nitrobenzene. N.Ndimethylaniline. and acetanilide the electronic absorption spectra indicate a complete or almost com,plete elimlnation of the mesomeric interaction. In contrast with this it has to be concluded for all the primary amines investigated that the mesomerism is fully developed: in the relevant discussion the vectorial treatment of absorption intensities has been of decisive importance. The secondary amines take up an intermediate position: in these cases, owing to the pyramidal configuration of the nitrogen atom, the angle of twist cannot be forced to values exceeding about 60'.T h e basic strength of 2,~i-tert.buty1-substituted primary amines is lower by a factor of about 108 than might be expected on the basis of the electrical effects of the alkyl groups: the basic strength of 2-methyl-6fert.buty1-substituted N,N-dirnethylanilines is lower by a factor of about 106 than might be expected on the basis of the electrical effects of the alkyl groups and the steric inhibition of the mesomerism present in these compounds. These reductions of the bask strength have been ascribed to steric hindrance to solvation.The basic strength of 3,5-di-tert.butyl-4-nitroaniline confirms previously obtained information on the quantitative significance of the inductive and the mesomeric effect of the nitro group in 4-nitroaniline as well as on the metalpara ratio of the inductive effect. ' 1) In the present paper, previous parts of this series are referred to as follows: la) Part 111. IS J7 19 41 4 J 45 I 100 280 260 24 0 2 Ware lrnglh, m),

Ultrafast Intramolecular Charge Transfer with Strongly Twisted Aminobenzonitriles: 4-(Di-tert-butylamino)benzonitrile and 3-(Di-tert-butylamino)benzonitrile

“…Amino Twist Angle of DTABN from Absorption Spectra. As can be seen from the data for a series of aminobenzonitriles and CBQ in n -hexane at 25 °C in Table , there is a clear correlation between the extinction coefficient ε max of the maximum of the lowest-energy absorption band and the amino twist angle θ determined from X-ray crystal analysis, as previously discussed in the literature. ,, By using this somewhat empirical correlation, a twist angle of 75° is calculated for DTABN from its ε max in n -hexane (Table ) . From a comparison of ε max for DTABN with that of DMABN in n -hexane (Table ), an amino twist angle θ of 78.6° is calculated, by employing the well-established relation cos 2 θ = ε max (DTABN)/ε max (DMABN) and θ = 0 for DMABN. ,, …”

Intramolecular Charge Transfer with the Planarized 4-Cyanofluorazene and Its Flexible Counterpart 4-Cyano-N-phenylpyrrole. Picosecond Fluorescence Decays and Femtosecond Excited-State Absorption

“…The twist angle θ between the pyrrole and benzonitrile moieties of PP4C in the ground state can be determined from the absorption coefficient ε max as compared with that (ε(ref)) of its planarized counterpart FPP4C as the reference compound with zero twist angle (Table ). This approach, based on eq , has been used to estimate θ for nonplanar 4-aminobenzonitriles, 1-aminonaphthalenes and N -phenylpyrroles in solution. ,,,– ε max / ε ( ref ) = cos 2 θ …”

Two-State Intramolecular Charge Transfer (ICT) with 3,5-Dimethyl-4-(dimethylamino)benzonitrile (MMD) and Its Meta-Isomer mMMD. Ground State Amino Twist Not Essential for ICT

“…Since 1959, the electron donor (D)/acceptor (A) molecules such as 4-(dimethylamino)­benzonitrile (DMABN) in the excited singlet state are well-known to undergo an intramolecular charge transfer (ICT) reaction, accompanied by the dual fluorescence from a locally excited (LE) and an ICT state. , In the initial discussions of these reactions, molecules considered to be model compounds for the LE and ICT states have played a prominent role. − As it was found that the planarized 1-methyl-5-cyanoindoline (NMC5) and later also 1-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (NMC6) , and 1-ethyl-6-cyano-1,2,3,4-tetrahydroquinoline (NEC6) only showed LE fluorescence, these molecules were adopted as model compounds for the LE state of DMABN. Within the context of the twisted ICT (TICT) concept, 3,5-dimethyl-4-(dimethylamino)­benzonitrile (MMD) was synthesized, with the expectation that the steric hindrance exerted by the two ortho methyl substituents would in the electronic ground state S 0 induce a substantial twist of the amino group relative to the phenyl ring . This indeed proved to be the case, with a twist angle later determined to amount to 57.4°.…”

“…3−6 As it was found that the planarized 1-methyl-5-cyanoindoline (NMC5) 3 and later also 1-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (NMC6) 4,5 and 1-ethyl-6-cyano-1,2,3,4-tetrahydroquinoline (NEC6) 6 only showed LE fluorescence, these molecules were adopted as model compounds for the LE state of DMABN. Within the context of the twisted ICT (TICT) concept, 3,5-dimethyl-4-(dimethylamino)benzonitrile (MMD) was synthesized, with the expectation 7 that the steric hindrance exerted by the two ortho methyl substituents would in the electronic ground state S 0 induce a substantial twist of the amino group relative to the phenyl ring. 8 This indeed proved to be the case, 8 with a twist angle later 9 determined to amount to 57.4°.…”