Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2018, 9, 4591−4595
Mid-Infrared Spectroscopy of C7H7+ Isomers in the Gas Phase: Benzylium and Tropylium J. Philipp Wagner, David C. McDonald, II, and Michael A. Duncan* Department of Chemistry, University of Georgia, 140 Cedar Street, Athens, Georgia 30602, United States
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S Supporting Information *
ABSTRACT: Both prominent C7H7+ isomers, the benzylium and the tropylium cations, were generated in an electrical discharge/supersonic expansion from toluene and cycloheptatriene precursors. Their infrared spectra were measured in the region of 1000−3500 cm−1 using photodissociation of the respective argon- and nitrogen-tagged complexes with a broadly tunable OPO/OPA laser system. Spectral signatures of both isomers were observed independent of the precursor, albeit in different relative intensities. The spectra were assigned based on scaled harmonic B3LYP-D3/cc-pVTZ frequency computations and comparisons to previous experimental studies. Consistent with its high symmetry, only two bands were observed for the (nitrogen-tagged) tropylium ion at 3036 and 1477 cm−1, corresponding to C−H stretching and C−C−H deformation/CC stretching vibrations, respectively. Furthermore, the C−H stretching region of the benzylium ion is reported for the first time. we present firm IR spectroscopic signatures for both the benzylium and tropylium ions. In initial mass spectrometric studies, the C7H7+ fragment ion was solely ascribed to the benzylium isomer, implying its direct formation from, for instance, the toluene radical cation.10 However, in 1957, Rylander, Meyerson, and Grubb suggested that C7H7+ displays a symmetric tropylium structure in order to account for the observed hydrogen scrambling when deuterated toluenes were employed.14 Nowadays, it is generally agreed that isomers 1 and 2 both explain the C7H7+ fragment ion in alkylbenzene mass spectra.10 The two isomers are usually distinguished based on their difference in reactivity; while the aromatic tropylium ion is unreactive, benzylium reacts with toluene by transfer of a CH2+ unit and formation of neutral benzene.6,9 In terms of spectroscopy, initial UV/vis studies of the benzylium ion were contradictory and reported either one or two electronic transitions.15−18 This was remedied in 2011 by Maier and co-workers, who provided well-resolved spectra of both C7H7+ isomers in a Ne matrix.19 The benzylium ion exhibits a weak transition in the 400−525 nm range with a distinct vibrational progression and a strong absorption in the 280−320 nm region. The tropylium isomer was observed as a very weak transition at around 275 nm. Subsequent gas phase photodissociation studies with and without argon tagging
H
ückel’s rule predicts aromatic character and concomitant stability for planar, cyclically conjugated structures with [4n + 2] π electrons.1,2 While benzene is the epitome of a compound with an aromatic sextet, formal insertion of a CH+ unit leads to the D7h symmetric tropylium cation 1 (Scheme 1). In agreement with the expected stability, tropylium was Scheme 1. Benzylium and Tropylium Isomers of the C7H7+ Cation
synthesized in 1954 as a bromide salt by Doering and Knox.3 Another familiar C7H7+ isomer is the benzylium ion 2 that exhibits a six-membered ring and is 7−11 kcal mol−1 less stable than 1 depending on the employed level of theory.4,5 The structural interconversion of benzylium to more stable 1 is possible via ring expansion and consecutive hydrogen shift and is associated with a barrier of about 65 kcal mol−1. Because fragment ions of this elemental composition are landmark features in the mass spectra of alkylbenzenes, they have been studied for several decades with mass spectrometry techniques.5−13 Despite the long-standing interest in these isomeric species, the spectroscopy of these isomers is still scarce, and their gas phase infrared spectra are still unknown. In this study, © XXXX American Chemical Society
Received: July 6, 2018 Accepted: July 30, 2018 Published: July 30, 2018 4591
DOI: 10.1021/acs.jpclett.8b02121 J. Phys. Chem. Lett. 2018, 9, 4591−4595
Letter
The Journal of Physical Chemistry Letters confirmed the matrix isolation experiment but only found evidence for the benzylium isomer.20,21 Infrared and Raman spectroscopy of the tropylium ion in KBr pellets and HBr solutions were reported just after the compound’s first synthesis.3,22,23 The spectra are rather simple and mutually exclusive, indicating high symmetry of the ion. For the benzyl cation, several fundamentals at frequencies below 1500 cm−1 are known from ZEKE photoelectron and threshold photoionization studies in the gas phase, although virtually none of the bands overlap those in our spectra.24,25 Many of the ZEKE bands were assigned to vibrational combinations not likely to be active in the infrared. Complementary information was obtained in an argon matrix isolation study in which 2 was formed from the corresponding radical through photoionization.26 The high frequency C−H stretches of benzylium were not reported in any of these studies. Gas phase infrared spectroscopy of the benzylium and tropylium structural motifs is known only for the methylated derivatives of 1 and 2.27 Here, we report vibrational spectroscopy of 1 and 2 by means of infrared photodissociation and utilization of argon and nitrogen messengers in the region of 1000−3500 cm−1. We learned after this work was completed that another study of these ions in the 600− 1650 cm−1 region was also ongoing.28 The experimentally observed infrared spectra of the C7H7+ ions formed from toluene and cycloheptatriene precursors and measured with the aid of argon (red) and nitrogen tagging (blue) are presented in Figure 1. Toluene is likely to favor the formation of the benzylium cation, whereas cycloheptatriene should favor the formation of tropylium. The spectra are similar and relatively rich in features pointing to a dominance of the less symmetric benzylium isomer for both precursors. However, with nitrogen tagging we observe a band at 1477
cm−1 that is completely absent when argon is used as the tag and a feature at 3036 cm−1 that appears only weakly at 3034 cm−1 with argon. This indicates the presence of two isomers, one of which is more difficult to tag with argon. Additionally, the relative intensity of these two bands is smaller with toluene as a precursor than that with cycloheptatriene. Because the spectra are very similar, we focus our discussion on the higher quality nitrogen tagging data, unless stated otherwise. The C− H stretching region is dominated by a group of four peaks at 2997, 3036, 3074, and 3116 cm−1 and a single one at 2799 cm−1. The strongest feature in the overall spectrum occurs as two overlapping peaks at 1615 and 1626 cm−1. In the fingerprint region, bands of medium intensity are observed at 1350, 1444, and 1477 cm−1 as well as weak ones at 1176, 1393, and 1418 cm−1. Unfortunately, some unavoidable negative excursions in the baseline of the argon-tagged spectrum are seen from strong absorptions one mass unit higher bleeding into the spectrum. To interpret these experimental findings, we optimized structures for 1 and 2 as well as their argon- and nitrogentagged isomers at the B3LYP-D3/cc-pVTZ level of theory and computed their vibrational spectra within the harmonic approximation. In agreement with previous theoretical studies,4,5 we found the tropylium structure to be more stable than benzylium by 8.7 kcal mol−1. All binding energies were predicted to be
