Compression in encapsulated carboxylic acid homodimers Demeter Tzeli, Ioannis D. Petsalakis, Giannoula Theodorakopoulos ⇑ Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Ave., Athens 116 35, Greece article info Article history: Received 19 February 2013 In final form 19 April 2013 Available online 27 April 2013 abstract A density functional theory study has been carried out on five carboxylic acid homodimers, free and encapsulated, in order to study the effect of encapsulation or compression. Attractive interactions between the guests and the walls of the capsules stabilize encapsulation even in cases of severe confine- ment where formation of dimers is unfavorable. Larger hydrogen bonds are calculated for the encapsu- lated than the isolated dimers except for the case of the bulkiest dimer considered, where encapsulation leads to shortening of the hydrogen bond. Shorter hydrogen-bond lengths with increasing size of the encapsulated dimers are calculated, in agreement with experimental work. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Observation of individual hydrogen bonded carboxylic acid di- mers in solution is difficult because of their nanosecond lifetimes and the rapid exchange of partners due to their weak interactions [1–3]. On the other hand, reversible encapsulation allows the tem- porary isolation by mechanical barriers at least for milliseconds and the characterization of the guest dimers by NMR methods at normal temperature and pressure conditions and in this manner it facilitates experimental investigations of hydrogen bonded carbox- ylic acid dimers [1,4]. The cage is considered to act as a solvent and it is of interest to examine the interactions between the guest mole- cules and the inner wall of the cages. The effect of encapsulation on different carboxylic acid dimers has been described in terms of ‘compressed hydrogen bonds’ [4], obtained from the observed NMR shifts and making use of established correlations between NMR parameters and hydrogen-bond lengths. Consequently, the ef- fect of attractive interactions of the guests with the inner walls of the cage is found to be comparable to the effect of external pressure on the geometries of carboxylic acid dimers in the solid state, caus- ing a decrease in the hydrogen-bond lengths with respect to that in the dimers in solution [4]. Furthermore a decrease of the hydrogen (O–HO) bond length in the encapsulated dimer with increasing size of the carboxylic acid is reported [4]. In the present study theoretical calculations are employed in or- der to determine the effect of three host cages on five carboxylic acid homodimers as guests, by analogy with the experimental work [4], from information on the encapsulated guest geometries and the degree of compression directly from the optimized geom- etries. Recently, the relative stability of encapsulated homodimers and heterodimers of amides carboxylic acids, and boronic acids in capsules have been examined both experimentally by NMR [1,5], and theoretically via DFT methodology in order to determine the % distribution of the encapsulated dimers [6–8]. The calculated and the experimental % distributions are in good agreement and it is shown that the size of the cage affects the % distribution. Such calculations are timely and important since theoretical insight in the reactivity and stability of encapsulated guests is required in view of the large number of experimental publications in the field and more generally in the field of chemistry in confined space. Gi- ven that processes such as selective recognition or catalysis of en- zyme binding pockets occur through hydrogen bonding, theoretical work on encapsulated complexes can provide useful information of what is involved. In addition to the cage employed in the experimental work, two smaller cages are employed here (see below) for examination of the effect of severe confinement on the dimers. 2. Computational procedure Five carboxylic acid dimers, namely benzoic acid, p-methyl-, p- ethyl-, p-vinyl-, and p-tert-butyl-substituted benzoic acid homodi- mers, here abbreviated as 4-H, 4-CH 3 , 4-CH 2 CH 3 , 4-CH@CH 2 , and 4-C(CH 3 ) 3 , respectively, are calculated in the gas phase and also as guests in reversible capsules. Of the above series p-methyl-, p- ethyl-, p-vinyl-, and p-tert-butyl-substituted benzoic acid homodi- mers along with the 12 4 .1 cage are included in the experimental study [4], see below. Three cages of different size or different sta- bility are employed in the present calculations, namely the 11, 3.3, and 12 4 .1 cages, see Figure 1. Cage 12 4 .1 consists of two cav- itands 1 with four glycouril spacer molecules 2 (see Figure 1), and as already mentioned it is the cage employed in the experimental study [4]. Encapsulated complexes of all five carboxylic acid di- mers in 12 4 .1 have been calculated. The smaller cages 11 (for encapsulated 4-H, 4-CH 3 , 4-CH 2 CH 3 ) and 3.3 ( (for 4-H), see Figure 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.04.043 ⇑ Corresponding author. Fax: +30 210 7273 794. E-mail address: ithe@eie.gr (G. Theodorakopoulos). Chemical Physics Letters 573 (2013) 48–55 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett