Can IR Absorption Peaks of Alkanes Be Predicted Based on Bond Strengths?

[hilbert2]

I have a work assignment where I'm supposed to calculate the transmittance of blackbody radiation through liquid n-heptane, and I have data of the heptane's absorption coefficient on the wavelength interval from 2.6 to 15 micrometers (it has significant peaks at about 3.5 $\mu$m, 7.0 $\mu$m and 14.0 $\mu$m). The problem is, a significant part of the blackbody radiative energy is at wavelengths under 2.6 $\mu$m at high temperatures and at wavelengths over 15 $\mu$m at lower temperatures. I managed to get a correct-looking result for high temperatures by assuming that there's no significant absorption maxima in the interval from visible light to 2.6 $\mu$m (of course there's none at visible range as it's a colorless liquid), but I'm not that sure about the long-wavelength range.

Is there some computer program or some simple approximative calculation that could predict, on the basis of the C-C and C-H bond strengths, the approximate IR wavelengths where an alkane hydrocarbon can have significant spectral peaks?

thanks,
Hilbert2

 

[DrClaude]

You can see the actual IR spectrum here: http://webbook.nist.gov/cgi/cbook.cgi?ID=C142825&Units=SI&Type=IR-SPEC&Index=2#IR-SPEC

 

[hilbert2]

Thanks, that one seems to have data up to 500 cm^-1 (or 20 $\mu$m wavelength), so it's better than what I had.

 

[DrClaude]

Thanks, that one seems to have data up to 500 cm^-1 (or 20 $\mu$m wavelength), so it's better than what I had.

That's basically the lower limit (in terms of energy) of vibrational absorption for organic molecules.

 

[hilbert2]

Ok, good, that solves the problem then.