  Determining the complexity of molecules has important applications from
molecular design to understanding the history of the process that led to the
formation of the molecule. Currently, it is not possible to experimentally
determine, without full structure elucidation, how complex a molecule is.
Assembly Theory has been developed to quantify the complexity of a molecule by
finding the shortest path to construct the molecule from building blocks,
revealing its molecular assembly index (MA). In this study, we present an
approach to rapidly and exhaustively calculate the MA of molecules from the
spectroscopic measurements. We demonstrate that molecular complexity (MA) can
be experimentally estimated using three independent techniques: nuclear
magnetic resonance (NMR), tandem mass spectrometry (MS/MS), and infrared
spectroscopy (IR), and these give consistent results with good correlations
with the theoretically determined values from assembly theory. By identifying
and analysing the number of absorbances in IR spectra, carbon resonances in
NMR, or molecular fragments in tandem MS, the molecular assembly of an unknown
molecule can be reliably estimated from experimental data. This represents the
first experimentally quantifiable approach to defining molecular assembly, a
reliable metric for complexity, as an intrinsic property of molecules and can
also be performed on complex mixtures. This paves the way to use spectroscopic
and spectrometric techniques to unambiguously detect alien life in the solar
system, and beyond on exoplanets.
