Determination of Molecular Formulas

It is often possible to derive the molecular formula of a compound by recording a high-resolution mass spectrum. As the atomic masses of the isotopes have non-integral masses (except 12C), the measurement of accurate masses to four to five decimal places allows compounds with different atomic compositions to be distinguished. 

From the molecular formula, the number of sites (or degrees) of unsaturation of an unknown molecule can also be calculated. This number is equal to the sum of the number of rings, the number of double bonds and twice the number of triple bonds. The sites of unsaturation for 

This compound is called 1-chloro-4-methoxybenzene. Naming compounds is discussed in Section 2.4 

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A double-focusing mass spectrometer can mea.sure mass accurately to several decimal places, thus enabling the determination of molecular formulae and elemental compositions of fragment ions. 

Regularities observed in the behavior of gases have contributed much to our understanding of the structure of matter. One of the most important regularities is Avogadro s Hypothesis Equal volumes of gases contain equal numbers of particles (at the same pressure and temperature). This relationship is valuable in the determination of molecular formulas—these formulas must be known before we can understand chemical bonding. 

Technological advances of ion-trap mass spectrometers are the ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and the recently released technique, the Orbitrap Fourier transform mass spectrometry (Hu et al., 2005), which enable the determination of molecular formulae with a high mass resolution and mass accuracy in mixtures. Today these ion-trap mass spectrometers are most frequently coupled with atmospheric pressure ionization (API) techniques such as electrospray ionization (ESI) (e.g., Fievre et al., 1997 Qian et al., 2001 Kujawinski et al., 2002 Llewelyn et al., 2002 Stenson et al., 2002,2003 Fard et al., 2003) or matrix-assisted laser desorption/ionization (MALDI) (e.g., Solouki et al.,... 

Natural Abundance of Important Isotopes Rules for Determination of Molecular Formula Neutral Moieties Ejected from Substituted Benzene Ring Compounds Order of Fragmentation Initiated by the Presence of a Substituent on a Benzene Ring... 

In 1893, at the age of 26, Alfred Werner stepped outside the box and proposed his coordination theory that revolutionized the held of inorganic chemistry. Werner s work, in part, made use of colorful cobalt compounds prepared by the reaction of ammonia with cobalt salts. Fortunately, these cobalt salts are quite stable ( robust ) in aqueous solution. This stability, along with the rapid development of qualitative and quantitative analytical techniques, enabled Werner to carry out the rigorous and in-depth analyses that led to the determination of molecular formulas and prediction of structures and geometries. His work opened the door to understanding bonding and stability (Chapter 3) and reactivity (Chapters 4 and 5) in metal coordination complex systems. 

Percent Composition and Formulas of Compounds 2-9 Derivation of Formulas from Elemental Composition 2-10 Determination of Molecular Formulas... 

Section 13.4 Determination of Molecular Formulas High-Resolution Mass Spectrometry... 

DETERMINATION OF MOLECULAR FORMULAS A. Precise Mass Determination... 

Double-focusing magnetic sector mass spectrometers and Fourier transform (FT) mass spectrometers (described in Chapter 9) are capable of mass measurements with high resolution. The most important use of high-resolution MS is the direct determination of molecular formulas by exact mass measurements. 

Determination of molecular formula candidates from exact molecular masses... 

In agreement with our previous considerations of MMD, an unambiguous determination of molecular formula is ensured for a precision of d > 5 decimal places for 4. For 11 there are cases in our sample where molecular formula determination is ambiguous even for 6 decimal places. In the extreme case there are 153 possible molecular formulas within this interval, although the 3rd quartile (Table 8.19) indicates that the majority of formulas can be unambiguously determined at 6 decimal place precision. 

M. Meringer, S. Reinker,]. Zhang, and A. Muller. MS/MS data improves automated determination of molecular formulas by mass spectrometry. MATCH Common. Math. Comput. Chem., 65 259-290, 2011. 

Chaki NK, Negishi Y, Tsunoyama H, Shichibu Y, Tsukuda T (2008) Ubiquitous 8 and 29 kDa gold alkanethiolate cluster compounds mass spectrometric determination of molecular formulas and structural implications. J Am Chem Soc 130 8608-8610. doi 10.1021/ja8005379... 

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