Molecular structure of caffeine as determined by gas electron diffraction aided by theoretical calculations

The molecular structure of caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) was determined by means of gas electron diffraction. The nozzle temperature was 185 °C. The results of MP2 and B3LYP calculations with the 6-31G∗∗ basis set were used as supporting information. These calculations predicted that caffeine has only one conformer and some of the methyl groups perform low frequency internal rotation. The electron diffraction data were analyzed on this basis. The determined structural parameters (rg and ∠α) of caffeine are as follows:  = 1.382(3) Å; r(CC) = 1.382(←) Å; r(CC) = 1.446(18) Å; r(CN) = 1.297(11) Å;  = 1.459(13) Å;  = 1.206(5) Å;  = 1.085(11) Å; ∠N1C2N3 = 116.5(11)°; ∠N3C4C5 = 121. 5(13)°; ∠C4C5C6 = 122.9(10)°; ∠C4C5N7 = 104.7(14)°; ∠N9–C4=C5 = 111.6(10)°; <∠NCHmethyl> = 108.5(28)°. Angle brackets denote average values; parenthesized values are the estimated limits of error (3σ) referring to the last significant digit; left arrow in parentheses means that this parameter is bound to the preceding one.