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

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.

There are several experiments and techniques that can be used as alternatives to infrared spectroscopy in various situations. Here are a few examples:

1. Nuclear Magnetic Resonance (NMR) Spectroscopy: NMR spectroscopy is a powerful technique that provides information about the structure and dynamics of molecules. It is particularly useful for studying the behavior of atomic nuclei in a magnetic field. NMR spectroscopy can be used to determine molecular structures, investigate chemical reactions, and analyze samples in a wide range of fields, including medicinal chemistry, biochemistry, and structural biology.

To perform NMR spectroscopy, you would typically subject a sample to a strong magnetic field and apply radiofrequency pulses to excite the atomic nuclei. By measuring the absorption or emission of energy by the nuclei, you can obtain information about the molecular structure and properties.

2. Mass Spectrometry: Mass spectrometry is a technique used to determine the molecular weight and composition of a sample. It can provide information about the elemental composition, isotope ratios, and the presence of impurities or contaminants in a compound. Mass spectrometry is widely used in fields such as organic chemistry, environmental analysis, forensics, and proteomics.

To conduct a mass spectrometry experiment, you would typically ionize the sample molecules, separate the ions based on their mass-to-charge ratio, and then detect and measure their abundance. This information can then be used to identify the molecular formula or structural features of the compounds in the sample.

3. Ultraviolet-Visible (UV-Vis) Spectroscopy: UV-Vis spectroscopy is a technique used to analyze the absorption or transmission of ultraviolet and visible light by a sample. It is particularly useful for studying the electronic transitions and color properties of compounds. UV-Vis spectroscopy finds applications in fields such as analytical chemistry, materials science, and biochemistry.

To perform a UV-Vis spectroscopy experiment, you would typically pass light of a specific wavelength through a sample and measure the absorption or transmission of the light. The resulting UV-Vis spectrum can provide information about the electronic structure and properties of the sample.

These are just a few examples of alternative experiments and techniques that can be used instead of infrared spectroscopy. The choice of the appropriate technique depends on the specific research question, the nature of the sample, and the type of information required.