The solid columnar discotic and liquid-crystalline phases formed by hexa-n-dodecyl-hexa-peri-hexabenzocoronene (HBC-C12) have been investigated by IR and Raman vibrational spectroscopies. IR spectra clearly show the two phase transitions at 42 and at 107 degrees C already reported in literature and allow us to understand the conformational modifications of the n-alkyl chains that take place through the transitions. Thanks to the collected data, we propose a model of the structure of HBC-C12 in the solid-crystalline phase below 42 degrees C which includes the structure of the alkyl chains. This model is also confirmed by dichroic infrared microscopy measurements on highly oriented samples.