Experimental ground-state combination differences of CH ₅ ⁺

Abstract

Protonation of methane (CH4), a rather rigid molecule well described by quantum mechanics, produces CH5(+), a prototypical floppy molecule that has eluded definitive spectroscopic description. Experimental measurement of high-resolution spectra of pure CH5(+) samples poses a formidable challenge. By applying two types of action spectroscopy predicated on photoinduced reaction with CO2 and photoinhibition of helium cluster growth, we obtained low-temperature, high-resolution spectra of mass-selected CH5(+). On the basis of the very high accuracy of the line positions, we determined a spectrum of combination differences. Analysis of this spectrum enabled derivation of equally accurate ground state-level schemes of the corresponding nuclear spin isomers of CH5(+), as well as tentative quantum number assignment of this enfant terrible of molecular spectroscopy.

Ultra-Low-Temperature Reactions of Carbon Atoms With Hydrogen Molecules

“…The CH 5 + was first detected in 1952, and since then it was a subject of intensive studies (Tal'roze & Lyubimova 1952, Asvany et al 2015, Ivanov et al 2010, White et al 1999, Sefcik et al 1974, Semaniak et al 1998, Thompson et al 2005. In spite of these thorough studies only the IR spectrum of this cation is currently available (Asvany et al 2005, Asvany et al 2015.…”

Structures and mechanisms in chemical reactions: George A. Olah’s life-long search of chemistry

“…, as recent as 2015, have suggested the preeminence of geometries fully consistent with Olah's description of the structure (Fig. 26) [19]. There is a caveat though, because all the spectroscopic evidence point to the highly fluxional character of this carbocation: ''the five proton swarm around the central carbon'' [20].…”

Oláh György (1927-2017) emlékezete

“…Ebben a vonatkozásban az NMR spektroszkópia a viszonylag hosszú kölcsönhatási idõ miatt hátrányban van a diffrakciós módszerekkel szemben, de a diffrakciós módszerek hátrányban vannak a hidrogénhelyzetek meghatározásának egyértelmûsége és más gyakorlati meggondolások szempontjából. Újabb nagyfelbontású spektroszkópiai mérések is a C s szimmetriájú szerkezetre utalnak, 4 de ugyanakkor erõs fluxionalitásra is, mintha az öt proton állandó mozgásban lenne a szén körül. 5 A CH 5 + karbokationnak hatalmas irodalma van, amely jóval elõbb kezdõdött, mint maga a karbokation elnevezés.…”