Abstract
The structural arrangement of small carboxylic acid molecules in the liquid phase remains a controversial topic. Some studies indicate a dominance of the cyclic dimer that prevails in the gas phase, whilst other studies favor short fragments of the infinite catemer chains that are found in the crystalline phase. Furthermore, difficulties in preparing and probing size-selected catemer segments have resulted in a lack of benchmark data upon which theoretical models of the condensed phases can be built. To address these issues, we have combined infrared spectroscopy and quantum chemical calculations to explore regions of the intermolecular potential energy surface associated with the formation of metastable dimer isomers. The OH stretching region of the spectrum shows that aggregation of acetic acid molecules inside liquid helium nanodroplets yields two distinct metastable dimers, whilst negligible signal is observed from the cyclic dimer that typically overwhelms this spectral region. We deduce that the most abundant isomer in superfluid helium has one strong O-HO[double bond, length as m-dash]C and one weak C-HO[double bond, length as m-dash]C hydrogen bond. Since this bonding motif is common to the dimeric repeating unit of the catemer, it is of fundamental importance for understanding intermolecular interactions in the condensed phases of small carboxylic acids.
Highlights
Dimers of carboxylic acids are classic examples of hydrogen bonded structures
The lowest energy structure of carboxylic acid homodimers has two strong and equivalent O−H···O=C hydrogen bonds, which yield the locally cyclic arrangement illustrated in Figure 1(a) for acetic acid (AA), and this cyclic structure is prevalent in the gas phase.[6,7]
The IR spectrum of AA dimers in helium nanodroplets is dramatically different from that recorded in the gas phase
Summary
Dimers of carboxylic acids are classic examples of hydrogen bonded structures. They are used as model systems to gain an understanding of proton donor-acceptor interactions, which are important in structural biology, molecular recognition and polymer science.[1,2,3,4,5] The lowest energy structure of carboxylic acid homodimers has two strong and equivalent O−H···O=C hydrogen bonds, which yield the locally cyclic arrangement illustrated in Figure 1(a) for acetic acid (AA), and this cyclic structure is prevalent in the gas phase.[6,7] the crystalline state of AA is dominated by catemer chains containing one strong O−H···O=C and one weak C−H···O=C bond between adjacent molecules, as shown in Figure 1(b), and the formation of extended structures is facilitated by weak C−H···O bonds between the catemer chains.[8,9]The structure of the liquid phase is still the subject of controversy, with some studies indicating a preponderance of cyclic dimers[10,11] whilst others favor segments of catemer chains,[12,13] potentially as short as the trimer.[14]. Dimers of carboxylic acids are classic examples of hydrogen bonded structures They are used as model systems to gain an understanding of proton donor-acceptor interactions, which are important in structural biology, molecular recognition and polymer science.[1,2,3,4,5] The lowest energy structure of carboxylic acid homodimers has two strong and equivalent O−H···O=C hydrogen bonds, which yield the locally cyclic arrangement illustrated in Figure 1(a) for acetic acid (AA), and this cyclic structure is prevalent in the gas phase.[6,7] the crystalline state of AA is dominated by catemer chains containing one strong O−H···O=C and one weak C−H···O=C bond between adjacent molecules, as shown, and the formation of extended structures is facilitated by weak C−H···O bonds between the catemer chains.[8,9]. This metastable dimer is the primary focus of the present work
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