Inner-phase reaction dynamics: The influence of hemicarcerand polarizability and shape on the potential energy surface of an inner-phase reaction

The thermal decomposition of six phenyldiazirine (2) hemicarceplexes and the spectroscopic properties of these hemicarceplexes, as well as those of one spiro[cyclobutabenzene-1(2H),3′-diazirine] (1) and one p-tolyldiazirine (3) hemicarceplex, have been investigated in order to determine the effect of a hemicarcerand on the potential energy surface of an inner-phase reaction. These hemicarceplexes have pentyl or phenethyl feet groups, three tetramethylene linkers and differ in the nature of one linker group X as follows: 1: X = (CH₂)₅; 2: X = (CH₂)_n (n = 2, 3, 4), (S,S)-CH₂CH[OC(CH₃) O]CHCH₂ and ortho-CH ₂C₆H₄CH₂; 3: X = (CH ₂)₄. The effect of the linker group X on the structure of the phenyldiazirine hemicarceplexes was analyzed by MM2 force field calculations and leads to a change in the bending of the inner phase, which increases in the order (S,S)-CH₂CH[OC(CH₃) O]CHCH ₂ > (CH₂)₄ > (CH₂) ₃ > (CH₂)₂ > ortho-CH₂C ₆H₄CH₂ and to a shortening of the center-to-center distance between the two cavitands of the host, which decreases in the order (S,S)-CH₂CH[OC(CH₃)O]CHCH ₂ < (CH₂)₄ < (CH₂) ₃ < (CH₂)₂ < ortho-CH₂C ₆H₄CH₂. All hemicarceplexes show large red shifts of the diazirine n-π* -transition in their UV/Vis absorption spectra. From the red shifts and from plots of the n-π* -excitation energy of the free diazirines against the solvent polarizability P, the inner-phase po larizability was estimated. P ranges from 0.39 to 0.58 and is larger than the polarizabilities of common organic solvents. A comparison of the activation parameters ΔH‡, TΔS‡, and ΔG‡ for the diazirine decomposition in the inner phase with those in the bulk phase shows that the hemicarcerand stabilizes the inner-phase transition states enthalpically by ΔΔH‡ = 1,9-2.8 kcal/mol and destabilizes the transition states entropically by ΔTΔS‡ = 0.2 to 3.0 kcal/mol. The enthalpic stabilization is explained with dispersion interactions between the stretched C-N bonds in the transition state and the highly electron-rich aryl units of the hemicarcerand, This is consistent with the high polarizability of the inner phase. The entropic destabilization of the inner-phase transition states is explained with a greater loss of vibrational degrees of freedoms as the transition state is reached in the inner phase as compared to the more mobile bulk solvent cage. Furthermore, the entropic destabilization decreases with an increased bending of the inner phase. This is explained with an induced-fit model. An increased hemicarcerand bending leads to an improved fit between the inner phase and the bend transition states, which reduces the loss of vibrational degrees of freedom as the transition states are reached.