what other experiments or techniques can be used in replace of infrared spectroscopy

Answer 1

It depends on why you are using ir.

If you are using ir as an analytical tool, i.e. to determine the concentration of something then there are likely to be other techniques, such as uv or wet chemistry, that may be as equally effective, but may be not as convenient.

If you are using ir as a tool to identify compounds then it may be difficult to find other techniques in some cases, although ir is rarely used on its own for this. As it tends to be used as an additional piece of evidence to confirm the presence of a functional group, then this information might be available from other techniques. For example the presence of a C=O (ketone) you could get from the C-13 nmr and a wet chemistry test, the C=O stretch in the ir would be confirmation only.

As a lecturer I used to set an exam question based around 'What evidence both chemical and physical is necessary and sufficient to establish structure...' and a third piece of evidence is not always needed.

Where ir is difficult to replace is comparing the ir of a complex structure with that in an ir library. This ir 'fingerprint' is unique for many molecues and provides indetification that can be hard to achieve other wise.