1,3,5-环庚三烯的气相碱度和质子亲合势——二氢环庚三烯正离子的能量学、结构及互变

Salpin Jean-Yves, Mormann Michael, Tortajada Jeanine, Nguyen Minh-Tho, Kuck Dietmar

Fakultät för Chemie, Universität Bielefeld, Universitätsstrasse 25, D-33615 Bielefeld, Germany.

Eur J Mass Spectrom (Chichester). 2003;9(4):361-76. doi: 10.1255/ejms.556.

The hitherto unknown gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene (CHT) have been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Several independent techniques were used in order to exclude ambiguities due to proton-induced isomerisation of the conjugate cyclic C(7)H(9)(+) ions, CHT + H. The gas-phase basicity obtained by the thermokinetic method, GB(CHT) = 799 +/- 4 kJ mol(-1), was found to be identical, within the limits of experimental error, with the values measured by the equilibrium method starting with protonated reference bases, and with the values resulting from the measurements of the individual forward and reverse rate constants, when corrections were made for the isomerised fraction of the C(7)H(9)(+) population. The experimentally determined gas-phase basicity leads to the proton affinity of cycloheptatriene, PA(CHT) = 833 +/- 4 kJ mol(-1), and the heat of formation of the cyclo-C(7)H(9)(+) ion, deltaH(f)(0)(CHT + H) = 884 +/- 4 kJ mol(-1). Ab initio calculations are in agreement with these experimental values if the 1,2-dihydrotropylium tautomer, CHT + H((1)), generated by protonation of CHT at C-1, is assumed to be the conjugate acid, resulting in PA(CHT) = 825 +/- 2 kJ mol(-1) and deltaH(f)(0)(300)(CHT + H((1))) = 892 +/- 2 kJ mol(-1). However, the calculations indicate that protonation of cycloheptatriene at C-2 gives rise to transannular C-C bond formation, generating protonated norcaradiene NCD + H, a valence tautomer being 19 kJ mol(-1) more stable than CHT + H((1)). The 1,4-dihydrotropylium ion, CHT + H((3)), generated by protonation of CHT at C-3, is 17 kJ mol(-1) less stable than CHT + H((1)). The bicyclic isomer NCD + H is separated by relatively high barriers, 70 and 66 kJ mol(-1) from the monocyclic isomers, CHT + H((1)) and CHT + H((3)), respectively. Therefore, the initially formed 1,2-dihydrotropylium ion CHT + H((1)) does not rearrange to the bicyclic isomer NCD + H under mild protonation conditions.

通过傅里叶变换离子回旋共振（FT - ICR）质谱法测定了迄今未知的1,3,5 - 环庚三烯（CHT）的气相碱度和质子亲和能。使用了几种独立技术，以排除由于共轭环状C(7)H(9)(+)离子CHT + H的质子诱导异构化所导致的模糊性。通过热动力学方法获得的气相碱度GB(CHT) = 799 ± 4 kJ mol⁻¹，发现在实验误差范围内，与从质子化参考碱开始用平衡法测量的值以及对C(7)H(9)(+)群体的异构化部分进行校正后，由正向和反向速率常数的单独测量得到的值相同。实验测定的气相碱度得出环庚三烯的质子亲和能PA(CHT) = 833 ± 4 kJ mol⁻¹以及环C(7)H(9)(+)离子的生成热ΔH(f)(0)(CHT + H) = 884 ± 4 kJ mol⁻¹。如果假设由CHT在C - 1处质子化生成的1,2 - 二氢卓鎓互变异构体CHT + H((1))为共轭酸，则从头算计算与这些实验值一致，得到PA(CHT) = 825 ± 2 kJ mol⁻¹和ΔH(f)(0)(300)(CHT + H((1))) = 892 ± 2 kJ mol⁻¹。然而，计算表明环庚三烯在C - 2处质子化会导致跨环C - C键形成，生成质子化降蒈二烯NCD + H，一种价互变异构体，比CHT + H((1))稳定19 kJ mol⁻¹。由CHT在C - 3处质子化生成的1,4 - 二氢卓鎓离子CHT + H((3))比CHT + H((1))不稳定17 kJ mol⁻¹。双环异构体NCD + H与单环异构体CHT + H((1))和CHT + H((3))分别由相对较高的势垒70和66 kJ mol⁻¹隔开。因此，在温和质子化条件下，最初形成的1,2 - 二氢卓鎓离子CHT + H((1))不会重排为双环异构体NCD + H。