Comparative study of the H-bond and FTIR spectra between 2,2-hydroxymethyl propionic acid and 2,2-hydroxymethyl butanoic acid.

In the present paper, the FTIR band and H-bond differences between the two dihydroxy-based carboxyls, 2,2-hydroxymethyl butanoic acid (hereinafter abbreviated to DMBA) and 2,2-hydroxymethyl propionic acid (hereinafter abbreviated to DMPA), were analyzed based on the crystal structure as well as the relation between H-bond and FTIR band of DMPA. In addition, the energy and length of the H-bonds formed between COOH, between OH, between COOH and OH were also calculated via molecular modeling. The results showed that three H-bond types existing in DMPA while only two in DMBA. The COOH pattern and H-bond type in DMBA and DMPA can be preliminarily judged according to the band position and relative strength of the nuOH and nuCOOH in FTIR spectrum. The H atom in COOH is a stronger H-bond donor than that in primary OH, while the O atom in primary OH is a stronger H-bond acceptor than that in COOH. The H-bond formed by two COOH is, therefore, weaker than that formed by COOH (as donor) and OH (as acceptor), which makes nuC=O shift to lower frequency in DMPA than in DMBA.