in the catemer and dimer motifs
Markos M. Papadakis*,1 and Roger A. Lalancette1
1Department of Chemistry,
Rutgers University, Newark New Jersey 07102
*Rutgers Undergraduate Research Fellow
Keywords: organic keto-carboxylic acids, hydrogen bonding
We have examined the x-ray structures of more than 60 organic keto-carboxylic acids in our laboratory to determine the nature of the hydrogen bonding between the molecules in the solid state. Keto-carboxylic acids, besides having a ketone function as part of the acid group, contain a second ketone elsewhere in the molecule. As these organic acids contain only one donor (the acidic H atom) and two possible acceptors (the carboxyl ketone, and the second functionality, another ketone), there are five different possibilities for hydrogen bonding: acid-to-acid dimer formation, acid-to-ketone catemer formation, intramolecular H bonding, acid-to-acid catemer formation and acid-to-ketone dimer formation. There is no way of predicting a priori which of these possible hydrogen bonding schemes a previously unstudied keto-carboxylic acid molecule will adopt. We have used single crystal x-ray structure analysis to find out which hydrogen bonding mode prevails in each molecule. Secondly, we also have used the CSD (Cambridge Structural Database) to analyze and compare all the other known keto-carboxylic acid structures in the literature.
Keto-carboxylic acids provide a useful model system to study simple hydrogen bonding. As each of these molecules contains one donor (the H atom) and two possible acceptors (the carboxyl C=O of the acid and the carbonyl C=O of the ketone), there are five different possible H-bonding motifs. Examples of these motifs are shown in Figures 1-5. Types 1 and 2 are the most common: 71 dimers (type-1) and 30 catemers (type-2) can be found in the literature. Types 3-5 are relatively rare. The structure of 1'-acetylferrocene-1-carboxylic acid, an example of the fifth type of H-bonding (acid-to-ketone dimer), has been completed in our laboratory recently7. We present correlation data showing the differences between the two most common types of H-bonding through a comparison of the 71 dimers with the 30 catemers (from the Cambridge Structural Database)1.
The following parameters relevant to hydrogen bonding in keto-carboxylic acids were extracted using ConQuest, a standard software package in the Cambridge Structural Database (CSD)1.
Bond angles: The bond angles theta and phi are defined as follows (Figure 6):
theta is the angle between the H-bond acceptor carbonyl and the hydrogen to which it is bonded. phi is the angle between the average plane of the H-bond accepting carbonyl and the hydrogen to which it is bonded.These angles together specify the location of the hydrogen in relation to the oxygen to which it is bound.
Bond distances: The following bond distances were obtained in Angstroms:
Donor-hydrogen to oxygen-acceptor distance.
A scatter plot (Figure 7) of angles theta vs. phi for all 71 dimers and 30 catemers shows that the values for dimers are clustered around means of theta = 113(7)° and phi = 5(5)°; for the catemers, the means are theta = 132(12)° and phi of 8(8)°, respectively. Some statistics for the data set are shown in Table 1. These results indicate that the hydrogen bonding in the dimers is more closely aligned to the ideal values of theta = 120° and phi = 0° than are those of the catemers.
The donor-hydrogen to oxygen-acceptor distances for the dimers and the catemers are shown in Figures 8 and 9, respectively. The mean value for the dimers is 1.73(15)Å and that for the catemers is 1.85(12)Å.
The H-bond distances (from oxygen-to-oxygen) are shown in Figures 10 and 11: the mean value for the dimers is 2.65(2)Å and that for the catemers is 2.71(6)Å.
Copyright 2002 by Roger A. Lalancette
Current URL: http://rutgersscholar.rutgers.edu/volume04/papalala/papalala.htm