The infrared (IR) and Raman spectra of eight substitutional carbon defects in silicon are computed at the quantum mechanical level by using a periodic supercell approach based on hybrid functionals, an all electron Gaussian type basis set and the CRYSTAL code. The single substitutional C s case and its combination with a vacancy (C sV and C sSiV) are considered first. The progressive saturation of the four bonds of a Si atom with C is then examined. The last set of defects consists of a chain of adjacent carbon atoms Curn:x-wiley:01928651:media:jcc26206:jcc26206-math-0001, with i = 1–3. The simple substitutional case, C s, is the common first member of the three sets. All these defects show important, very characteristic features in their IR spectrum. One or two C related peaks dominate the spectra: at 596 cm−1 for C s (and C sSiV, the second neighbor vacancy is not shifting the C s peak), at 705 and 716 cm−1 for C sV, at 537 cm−1 for Curn:x-wiley:01928651:media:jcc26206:jcc26206-math-0002 and Curn:x-wiley:01928651:media:jcc26206:jcc26206-math-0003 (with additional peaks at 522, 655 and 689 for the latter only), at 607 and 624 cm−1, 601 and 643 cm−1, and 629 cm−1 for SiCurn:x-wiley:01928651:media:jcc26206:jcc26206-math-0004, SiCurn:x-wiley:01928651:media:jcc26206:jcc26206-math-0005, and SiCurn:x-wiley:01928651:media:jcc26206:jcc26206-math-0006, respectively. Comparison with experiment allows to attribute many observed peaks to one of the C substitutional defects. Observed peaks above 720 cm−1 must be attributed to interstitial C or more complicated defects.