Dihydrogen Bonding-Seen through the Eyes of Vibrational Spectroscopy

Abstract

In this work, we analyzed five groups of different dihydrogen bonding interactions and hydrogen clusters with an H3+ kernel utilizing the local vibrational mode theory, developed by our group, complemented with the Quantum Theory of Atoms-in-Molecules analysis to assess the strength and nature of the dihydrogen bonds in these systems. We could show that the intrinsic strength of the dihydrogen bonds investigated is primarily related to the protonic bond as opposed to the hydridic bond; thus, this should be the region of focus when designing dihydrogen bonded complexes with a particular strength. We could also show that the popular discussion of the blue/red shifts of dihydrogen bonding based on the normal mode frequencies is hampered from mode-mode coupling and that a blue/red shift discussion based on local mode frequencies is more meaningful. Based on the bond analysis of the H3+(H2)n systems, we conclude that the bond strength in these crystal-like structures makes them interesting for potential hydrogen storage applications.

Keywords: blue/red shifts; dihydrogen bonding; hydride complexes; hydrogen storage; local vibrational mode analysis.

Figure 2

(a) BSO n values for DHB, (b) BSO n values for protonic parts of DHBs, and (c) BSO n values for hydridic parts of DHBs calculated from the corresponding local mode force constants as described in the text. (d) Correlation between $k^a$(Protonic) and $k^a$(Hydridic), (e) between $k^a$(Protonic) and $k^a$(DHB), (f) between $k^a$(Hydridic) and $k^a$(DHB).

Figure 3

(a) Correlation of $k^a$(DHB) and normalized energy density $H_c/{\rho }_c$. (b) Correlation of charge differences $\Delta q$ (see Table 1 for definition) and $k^a$(DHB).

Figure 4

(a) Correlation between binding energy E${}_{bin}$ scaled by the number of DHBs n and $k^a$(DHB), (b) correlation between distance R(DHB) and $k^a$(DHB), (c) correlation between protonic bond angle (X–H${}^{\delta +}\cdots$H${}^{\delta -}$) and $k^a$(DHB), (d) correlation between hydridic bond angle (H${}^{\delta +}\cdots$H${}^{\delta -}$–Y) and $k^a$(DHB).

Figure 5

(a) Local mode frequency shifts $\Delta \omega$ of the protonic parts of DHBs. (b) Local mode frequency shifts $\Delta \omega$ of the hydridic parts of DHBs, where a positive sign denotes a blue–shift and a negative sign a red–shift.

Figure 6

(a) CNM analysis for the BeO3 complex. (b) CNM analysis for the two reference molecules HOCL2 (DHB donor) and BeH (DHB acceptor) forming the complex.

Figure 7

(a) Correlation between local mode frequency shifts $\Delta \omega$(Hydridic) and $\Delta \omega$(Protonic). (b) Correlation between the relative frequency shifts $\Delta \omega$/$\omega$(Hydridic) and $\Delta \omega$/$\omega$(Protonic). (c) Correlation between $\Delta \omega$/$\omega$(Protonic) and $k^a$(DHB). (d) Correlation between $\Delta \omega$/$\omega$(Hydridic) and $k^a$(DHB).

Figure 8

(a) Correlation between E${}_{bin}$/n and $\Delta \omega$/$\omega$(Protonic). (b) Correlation between E${}_{bin}$/n and $\Delta \omega$/$\omega$(Hydridic). (c) Correlation between $H_c/{\rho }_c$(Protonic) and $\Delta \omega$/$\omega$(Protonic). (d) Correlation of $H_c/{\rho }_c$(Hydridic) and $\Delta \omega$/$\omega$(Hydridic).

Figure 9

(a) BSO n(HH) as a function of the local mode force constant k${}^a$(HH); power relationship based on H${}_2$ and H${}_2^{+}$ references see text. (b) Correlation of distance R(HH) and k${}^a$(HH). (c) Correlation of k${}^a$(HH) and normalized energy density $H_c/{\rho }_c$.

Figure 1

Sketches of Group I–Group VI members and reference compounds investigated in this study. DHB distances (in Å) are given in blue. Short names of the complexes used throughout the manuscript are given in black.

Figure 1

Sketches of Group I–Group VI members and reference compounds investigated in this study. DHB distances (in Å) are given in blue. Short names of the complexes used throughout the manuscript are given in black.