Comment on “Arresting an Unusual Amide Tautomer Using Divalent Cations”

Abstract

Metrics & MoreArticle Recommendations * sı Supporting InformationH erein, we comment on the of spectroscopic data in "Arresting an Unusual Amide Tautomer Using Divalent Cations", J. Phys. Chem. B 2019, 123 (40), 8419−8424, by Bagchi and co-workers. 1 In their paper, the authors assign the appearance of a blue-shifted shoulder on the amide I vibration of N-methylacetamide (NMA), N,Ndimethylacetamide (DMA), and urea to the formation of a tautomer-like structure in the presence of divalent metal cations. In other words, the shoulder should originate from a CN + resonance that is stabilized by the presence of divalent metal cations. Moreover, Bagchi and co-workers argue that the CN + resonance is coincidently located at a position just to the blue of the amide I band upon the introduction of divalent metal cation salts. This assignment represents an alternative to the idea that the blue-shifted shoulder on the amide I band arises from the dehydration of the amide oxygen upon interaction with metal cations. 2,3 Bagchi's putative new assignment can be directly tested by studying 15 N-isotopelabeled NMA. Such experiments were performed in our laboratory and are described below.We synthesized NMA and 15 N-labeled NMA by reacting acetyl chloride with methylamine•HCl and 15 N-labeled methylamine•HCl, respectively (see Supporting Information for details). The resulting amide-containing molecules were introduced into D 2 O at a concentration of 100 mM (Figure 1A). In the absence of salt, there is a small isotope shift upon substitution with the 15 N-labeled molecule. This very modest red shift from 1620 to 1617 cm −1 in the case of 15 N-labeled NMA is expected because the amide I resonance is not purely CO in character. Upon adding 4 M CaCl 2 , a blue-shifted shoulder appears at 1645 cm −1 with NMA as well as with 15 Nlabeled NMA (Figure 1B). This is not what one would expect if the blue shoulder was from CN + . Indeed, if the CN + assignment was correct, then this mode should have been quite sensitive to the mass of the nitrogen atom. Moreover, one can look for an isotope shift upon the addition of acid to solution. As Bagchi and co-workers point out, one expects to observe the formation of a CN + resonance at low pH. As such, we added 1 M DCl to solution to directly examine this feature (Figure 1C). In the case of NMA, a new peak appears at 1680 cm −1 (Figure 1C, black curve). Next, the same experiment was performed with a solution containing 15 N-labeled NMA

Local Electric Fields in Aqueous Electrolytes

“…Previous studies have, however, shown that the introduction of certain cations can lead to a new resonance that shows up as a blue shoulder approximately 20–25 cm –1 above the frequency of the main carbonyl resonance (i.e., in the case of amides). ,, This happens when the cation contact pairs with the CO oxygen atom and dehydrates it (Figure S2). As such, CO···H 2 O hydrogen bonds are displaced . By contrast, the continuous Stark shift in the hydrated bond resonance toward lower frequencies should arise from water-separated ions that act via a through space electric field.…”

“…Previous studies have, however, shown that the introduction of certain cations can lead to a new resonance that shows up as a blue shoulder approximately 20−25 cm −1 above the frequency of the main carbonyl resonance (i.e., in the case of amides). 47,49,56 This happens when the cation contact pairs with the CO oxygen atom and dehydrates it (Figure S2). 56 As such, CO••• H 2 O hydrogen bonds are displaced.…”

“…47,49,56 This happens when the cation contact pairs with the CO oxygen atom and dehydrates it (Figure S2). 56 As such, CO••• H 2 O hydrogen bonds are displaced. 57 By contrast, the continuous Stark shift in the hydrated bond resonance toward lower frequencies should arise from water-separated ions that act via a through space electric field.…”

“…Herein, it was shown that the vibrational resonance of carbonyl and nitrile probes shows a Stark shift to the red upon introduction of salt to solution. This is in contrast to the blue-shifted shoulder that has been previously reported for cation binding events in which the probe is dehydrated (e.g., Ca 2+ interaction with NMA). , The molecular origins of this red-shifted Stark effect are discussed below.…”

“…This is in contrast to the blueshifted shoulder that has been previously reported for cation binding events in which the probe is dehydrated (e.g., Ca 2+ interaction with NMA). 49,56 The molecular origins of this redshifted Stark effect are discussed below.…”