Protoniertes Benzol: IR-Spektrum und Struktur von C6H7

Abstract

Die Protonierung aromatischer Molek¸le ist ein zentraler Prozess in der Organischen Chemie. Beispielsweise treten protonierte aromatische Molek¸le (AH þ ) als Intermediate in elektrophilen aromatischen Substitutionsreaktionen auf, dem wahrscheinlich typischsten Reaktionsmechanismus aromatischer Molek¸le. [1] Spektroskopische Studien in Lˆsung zeigen, dass fundamentale Eigenschaften dieser Ionen-Molek¸l-Reaktionen stark von der Umgebung abh‰ngen. [1, 2] Um Solvatationseffekte von den intrinsischen molekularen Eigenschaften zu separieren, sind Studien in der Gasphase erforderlich. [3] Fast alle bisher bekannten Daten zu Protonierungsprozessen in der Gasphase wurden mit massenspektrometrischen Methoden gewonnen, [3] die jedoch nur indirekte und h‰ufig nicht eindeutige Strukturinformationen liefern. Spektroskopische Daten f¸r die direkte und eindeutige Strukturbestimmung sind bisher f¸r isolierte AH þ -Ionen nicht erh‰ltlich, haupts‰chlich wegen der Schwierigkeit, hohe Konzentrationen zu erzeugen. Spektren von AH þ in der Gasphase sind nicht nur w¸nschenswert, um grundlegende Reaktionsmechanismen zu studieren, sondern auch, um diese Ionen in terrestrischen und extraterrestrischen Kohlenwasserstoffplasmen zu identifizieren, z. B. in Verbrennungsprozessen [4] und interstellaren Medien. [5] Protoniertes Benzol, C 6 H 7 þ ZUSCHRIFTEN

Superacidity of Boron Acids H2(B12X12) (X=Cl, Br)

“…In particular, the n(CH 2 ) frequencies of the most acidic CH 2 group at the protonated sp 3 carbon atom appear as a broad absorption centered at 2733 cm À1 for X = Cl and 2664 cm À1 for X = Br. These frequencies are lower than those in the gas phase (2810, 2795 cm À1 ) [14] owing to H-bonding to halide atoms of the anions. The greater red shift for the bromide is consistent with the higher basicity of the B 12 Br 12 2À ion relative to the B 12 Cl 12 2À ion.…”

The Electrophilic Substitution of Arenes: Is the π Complex a Key Intermediate and What Is Its Nature?

“…Chemie metrical over-ring structure for the complex from mesitylene and NO in 1991. [7b] The corresponding CT and s complexes have also been investigated experimentally and theoretically for the reaction of arenes with other electrophiles, such as H þ , [10] secondary and tertiary alkyl cations, [11] and SO 3 . [12] The existence of a symmetrical overring structure can be excluded in these cases as energetically it lies much higher than the other configurations.…”

“…In the case of protonated benzene, C 6 H 7 þ , the s complex can be detected as the most stable species with high-resolution IR spectroscopy (band analysis of the CÀH vibration at 2800 cm À1 ) by the photodissociation of the complexes formed with argon or nitrogen in the gas phase. [10] This result agrees with calculations according to which the over-edge protonated form and the symmetric over-ring structure are more energy rich, by 6.4 kcal mol À1 and 49.3 kcal mol À1 , respectively, than the benzenium ion structure. This result also agrees with the NMR spectroscopic investigations of Olah et al who have investigated this species in superacid solution.…”

Observation of an Isolated Intermediate of the Nucleophilic Aromatic Substition Reaction by Infrared Spectroscopy

“…In such systems, ionization causes the aromatic compounds to become highly electron deficient, thus ensuring the high reaction efficiencies. Recently, several σ‐complex‐type structures, such as H + C 6 H 6 , H + C 6 H 5 F, (C 6 H 6 ‐NH 3 ) + , and [CH 3 OC 6 H 3 (NO 2 ) 3 ] − have been observed in the gas phase by spectroscopy 11–15. However, the elimination step is unfavorable in these cases because of their poor leaving group (X=H).…”