As has been repeatedly stated, a permanent dipole moment is not necessary for IR activity. What is required is a dipole moment that changes during the vibration (even if the time-average is zero). Heating will not affect the dipole moment significantly, but it may affect the spectrum by populating higher rotational and vibrational states (that's one way of measuring the temperature of a gas).
Did you not do some homework before buying a Raman scanner, and realise that water is a bad Raman scatterer? That's one of the advantages of Raman - you can look at species in aqueous solution , which you can't with IR because of the massive absorbance of water.
I'm assembling a molecular laboratory. I need to acquire an IR Spectrometer. But the cheapest ones available (in fact mostly available) are the NIR spectrometer with range of wavelength of 650 to 2500 nm. The fundamental IR active modes are all above 2500nm. The ones below are just the overtones. I want to ask about overtones and how any external energy introduced into the molecules can show up in the spectrums from 650 to 2500nm.
2,700 nm corresponds to about 3700 cm-1
15,000 nm corresponds to about 650 cm-1
In details. NIR spectrometers have primarily a range of 900nm to 2400nm (or 11,111 cm-1 to 4166 cm-1). In the following table or list of the molecules functional groups, the range is between 2700nm to 15,000 nm (3700 cm-1 and 650 cm-1). Outside of the range of NIR spectrometers.
the rest the list is in the url above.
So if I get an IR spectrometers that can only detect the overtones of the fundamental IR dipole modes. And external energy is introduced into the molecules. How much will it show up in the 650 to 2500 nm wavenength range? How much can external energy affect the overtones? Or not visible at all? I won't get NIR Spectrometers to study functional groups but just to see if the overtones can be affected by any external energy. What is the wavelengh of higher rotational and vibrational states?