Infrared spectra of argon matrix-isolated alkali halide salt/water complexes

Ault, Bruce S.

doi:10.1021/ja00476a027

Cited by 190 publications

(145 citation statements)

References 9 publications

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“…18 These bands are reasonably close to those observed in our work if we allow for a redshift of 20-30 cm −1 , which is certainly possible. 26 Note also that the higher frequency band in the matrix experiment is much stronger than the lower frequency band, which again agrees with our findings.…”

supporting

confidence: 87%

“…The first spectroscopic study of the simplest complex, NaCl(H 2 O), used co-deposition of NaCl and water into an argon matrix. 18 Infrared (IR) spectra revealed peaks in the OH stretching region which were assigned to NaCl(H 2 O). Since that early work, the only other spectroscopic study of NaCl(H 2 O) n complexes was by Endo and co-workers, who obtained microwave spectra for n = 1-3.…”

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confidence: 99%

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Communication: Infrared spectroscopy of salt-water complexes

Tandy

Cheng

Boatwright

et al. 2016

The Journal of Chemical Physics

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To explore how the ion-pair in a single salt molecule evolves with the addition of water, infrared (IR) spectra of complexes composed of NaCl and multiple water molecules have been recorded for the first time. The NaCl(H2O)n complexes were formed and probed in liquid helium nanodroplets, and IR spectra were recorded for n = 1 → 4. The spectra for n = 1, 2, and 3 are consistent with formation of the lowest energy contact-ion pair structures in which each water molecule forms a single ionic hydrogen bond to an intact Na+Cl− ion-pair. Alternative structures with hydrogen bonding between water molecules become energetically competitive for n = 4, and the IR spectrum indicates likely the coexistence of at least two isomers.

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supporting

confidence: 87%

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confidence: 99%

Communication: Infrared spectroscopy of salt-water complexes

Tandy

Cheng

Boatwright

et al. 2016

The Journal of Chemical Physics

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“…For NaCl(CH 3 OH) 2 , we have found five distinct minima (see Figure 4). The lowest-energy structure is analogous to the n = 1 complex but now has two IHBs linking the chloride ion and the OH groups.…”

Section: Predicted Structuresmentioning

confidence: 88%

“…Second, the relatively narrow OH stretching bands sit astride a broad underlying absorption feature, which we attribute to complexes with relatively large values of n (≥4). This quasi-continuous absorption is very obvious in the (CH 3 OH) 2 Na + and (CH 3 OH) 3 Na + mass channels in Figure 2, but there is even a contribution in the (CH 3 OH)Na + channel, and this grows as the partial pressure of methanol is increased. We can partially offset the effect of the broad absorption envelope by estimating and subtracting its contribution from the intensities of the narrower peaks as the methanol partial pressure is changed.…”

Section: Predicted Structuresmentioning

confidence: 94%

“…19,20 Clearly, there are some expected similarities between the interactions of methanol with Cl − and NaCl given that the latter is a strongly ionic molecule, and therefore, we compare key features of their spectra here. For Cl − (CH 3 OH) 2 , two distinct structures were identified from the IR spectra: (1) one in which each methanol molecule is bound to Cl − by an IHB and (2) a "two-shell" structure in which a methanol molecule binds to Cl − by an IHB and then the second methanol molecule binds to the first by a normal hydrogen bond. The latter structure delivers a cooperatively enhanced IHB and results in an intense and hugely red-shifted band for this OH group, which appears at 2773 cm −1 .…”

Section: The Journal Of Physical Chemistry a Articlementioning

confidence: 99%

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Infrared Spectroscopy of NaCl(CH₃OH)_n Complexes in Helium Nanodroplets

et al. 2016

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Infrared (IR) spectra of complexes between NaCl and methanol have been recorded for the first time. These complexes were formed in liquid helium nanodroplets by consecutive pick-up of NaCl and CH 3 OH molecules. For the smallest NaCl(CH 3 OH) n , complexes where n = 1−3, the IR data suggest that the lowest-energy isomer is the primary product in each case. The predominant contribution to the binding comes from ionic hydrogen bonds between the OH in each methanol molecule and the chloride ion in the NaCl, as established by the large red shift of the OH stretching bands compared with the isolated CH 3 OH molecule. For n ≥ 4, there is a dramatic shift from discrete vibrational bands to very broad absorption envelopes, suggesting a profound change in the structural landscape and, in particular, access to multiple low-energy isomers. ■ INTRODUCTIONAlkali halides (MX) are archetypal univalent salts. The crystalline structure of the solid is disrupted by water, and these salts readily dissolve to form separated ions in dilute aqueous solutions. If the solid represents one extreme, then the counterpart is an isolated MX molecule. How might this smallest entity of salt, a single MX molecule, behave in the presence of a small water cluster, (H 2 O) n ? This problem can be addressed in experiments by forming isolated MX(H 2 O) n complexes and then using techniques, such as spectroscopy, to determine their properties. Such studies offer insight into the interaction between the constituent ionic particles and the solvent and can be reinforced through quantum chemical calculations. Understanding these interactions and the balance between them is critical in being able to carry out accurate simulations of bulk solutions. An early infrared (IR) spectroscopic study of a NaCl/H 2 O mixture in an argon matrix provided a tentative assignment of vibrational bands of the NaCl(H 2 O) complex.2 The first detailed spectroscopic study of several different NaCl(H 2 O) n complexes (n = 1−3) was obtained by microwave spectroscopy, and this revealed important structural information. 3,4 We have recently shown that complexes between NaCl and water molecules can be formed inside liquid helium nanodroplets, and this made it possible to record the first IR spectra of several small NaCl(H 2 O) n complexes. 5 The spectra for n = 1−3 are consistent with structures in which an ionic hydrogen bond (IHB) is formed between a single OH group in each water molecule and the Cl − in the salt. Evidence for interwater hydrogen bonding only begins to appear for n ≥ 4.An alternative solvent to water is methanol. Most solid alkali halides will dissolve in methanol, but their solubility is far lower than that in water. 6,7 The reduced solubility of salts in methanol when compared with that in water is a consequence of the hydrophobic (CH 3 ) group in the former. It would therefore be interesting to investigate how this amphiphilic character affects the interactions between a single salt molecule and one or more methanol molecules. It is well-known that...

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