Prediction of conformational population of large cycloalkanes using ab initio correlated methods: Cycloundecane, cyclododecane, and cyclotridecane

We report MP4(SDTQ) and CCSD(T) ab initio temperature-dependent conformational populations for a series of cycloalkanes with increasing ring size, cycloundecane (11-C), cyclododecane (12-C), and cyclotridecane (13-C), using extended basis sets. Our aim is to analyze the commonly used quantum chemical procedure for evaluating the thermal energy contribution to Gibbs free energy particularly the adequacy of standard harmonic oscillator vibrational partition function for large molecular systems. Experimental population data for cycloundecane, obtained from low temperature (90.1 K) 13C NMR, and cyclododecane, from gas phase electron diffraction experiment at 393.15 K, are available, and we found a good agreement with MP4(SDTQ) and CCSD(T) ab initio calculated conformational population. Our results indicate that the procedure of considering all 3N-6 vibrational modes as harmonic oscillators seems adequate for the prediction of the preferred conformer of cycloundecane, cyclododecane, and cyclotridecane, in the absence of a more appropriate approach to treat the low frequency modes that is applicable for large molecules. Therefore, it can be expected to work satisfactorily in room temperature conformational analysis studies on large molecules containing a great number of vibrational frequencies below 625 cm−1, such as in supramolecular chemistry, biological, and material science applications. © 2012 Wiley Periodicals, Inc.