Tagging effects on the mid-infrared spectrum of microsolvated protonated methane

Abstract

Although bare protonated methane is by now essentially understood at the level of intramolecular large-amplitude motion, scrambling dynamics and broadband vibrational spectra, the microsolvated species still offer plenty of challenges.

Comment on “Inter/Intramolecular Bonds in TH5+ (T = C/Si/Ge): H2 as Tetrel Bond Acceptor and the Uniqueness of Carbon Bonds”

“…Last but not least, we consistently reported no vibrational signatures of a putative H–H bond which, if existing, would show up as a H–H stretch in vibrational spectra. Instead, we demonstrate exactly the opposite of that picture: “Thus, the protons forming the H 2 moiety and those enganged in the CH 3 tripod lead to three well-separated stretching peaks in the low-temperature regime, the prominent one of highest frequency (tripod modes) having a triplet substructure” and “In particular, the fact that the C–H stretching modes of the H 2 moiety and CH 3 tripod resolve into distinct peaks is arguably an experimental support for three-center two-electron bonding being operative in bare CH 5 + under experimental conditions.” That absence of any vibrational fingerprint of a direct H–H bond is also consistent with all our subsequent publications that report vibrational spectra of CH 5 + species. − Importantly, the largely decoupled H 2 stretches of CH 5 + complexed with additional H 2 molecules can be nicely observed in our IR spectra (Figure 5) of these tagged CH 5 + ···(H 2 ) n species …”

“…+ complexed with additional H 2 molecules can be nicely observed in our IR spectra (Figure 5) of these tagged CH 5 7 Therefore, only significantly misrepresenting earlier literature 2,4 allows the authors to draw their key conclusion as to protonated methane (see abstract and conclusions: "Hence, the popular characterization of protonated methane as a weakly bound CH 3 + and H 2 is inconsistent with these results" and "We conclude that the common description of CH 5 + as a complex between CH 3 + and H 2 is not consistent with all the results presented here"), which was never concluded in the literature 2,4 cited in ref 1 to support this inconsistency or controversy. It is reassuring to see that the authors corrected these previous statements 1 in their Reply manuscript to my Comment as follows: "... we agree that Marx and others have not described CH 5 + as a complex between CH 3 + and H 2 ..." and "... we agree with Marx that there is a three-centre-two-electron bond ...".…”

Do Stereochemical Effects Overcome a Charge-Induced Perturbation in Isolated Protonated Cyclo(Tyr-Tyr)?

“…However, the spectral patterns themselves did not change significantly in agreement with recent results, suggesting that H 2 tagging only perturbs the spectrum for highly symmetrical systems such as CH 5 + . 60 It should be noted that the ν(OH + ) is up-shifted by the H 2 tag, in contrast to the other modes that are downshifted. Moreover, the two phenolic ν(OH) undergo similar upshift upon tagging as do the two amide ν(CH).…”

Vibrational spectroscopy by means of first‐principles molecular dynamics simulations

“…127 The same approach has been applied for the mode assignment in the H 2 tagged CH 5 + ion using MLWFs and was called the maximum entropy method. 129 Another way of separating the spectroscopic signals was applied by Luber and is based on periodic subsystem DFT. In this approach, the ill-definition of the position operator in periodic systems can be circumvented by defining subsystems in the total investigated system efficiently using atom-centered basis functions.…”

“…In this way, they were able to split the dipole–dipole correlation function into autocorrelation and cross‐correlation terms, elucidating which modes are coupled and which modes are independent 127 . The same approach has been applied for the mode assignment in the H 2 tagged CH 5 + ion using MLWFs and was called the maximum entropy method 129 …”