Molecular formula, determination by mass spectrometry

Amongst products isolated from Heliotropium spathulatum (Boraginaceae) were 9 mg of a new alkaloid which gave a positive Ehrlich reaction with p-dimethylaminobenzaldehyde The molecular formula determined by mass spectrometry is CisH2sNOs. What is the structure of the alkaloid given the set of NMR results 54 Reference is useful in providing the solution to this problem. Conditions CDCI3, 9 mg per 0.3 ml, 25 °C, 400 MHz ( //), 100 MHz ( C). (a) HH COSY plot ... 

The previously determined (38) molecular formula was corrected to C16H22N4O3 by quantitative elemental analysis and a molecular weight determination by mass spectrometry. 

A compound with the molecular formula C7///JNO3, determined by mass spectrometry, was isolated from the plant Petiveria alliacea (Phytolaccaceae). What is its structure given the set of NMR results 52 ... 

The molecular formula of a compound shows the actual number of each kind of atom present in a molecule of the compound. To work out the molecular formula of a compound, we need to know both the empirical formula and the relative molecular mass of the compound. The latter can be determined by mass spectrometry (see p. 73). 

The chemical makeup of a substance is described by its percent composition—the percentage of the substance s mass due to each of its constituent elements. Elemental analysis is used to calculate a substance s empirical formula, which gives the smallest whole-number ratio of atoms of the elements in the compound. To determine the molecular formula, which may be a simple multiple of the empirical formula, it s also necessary to know the substance s molecular mass. Molecular masses are usually determined by mass spectrometry. 

An organic compound was prepared containing at least one and no more than two sulfur atoms per molecule. The compound had C and H, no N, but O could have been present. The molecular mass of the predominant nuclidic species was 110.020 u, as determined by mass spectrometry. What is the molecular formula of the compound ... 

A substance with the molecular formula CnH2o04, as determined by mass spectrometry, was isolated from the light petroleum extract of the Chilean medicinal plant Centaurea chilensis (Compositae) 8 mg were available for the set of NMR experiments 45. Beyond the shift range shown in (c), the C NMR spectrum shows the signals of quaternary C atoms at 170.1,169.2,149.8,142.9 and 137.5 ppm. A CH COLOC plot was not recorded owing to shortage of material and time. It was nonetheless possible to identify the natural product which was already known. What is its structure ... 

The molecular formula of villalstonine, earlier presumed to be C40H50N4O4, was subsequently corrected to C41H48N4O4 on the basis of the molecular weight (660) determined by mass spectrometry (28). 

At one time, the molecular weight of a compound was determined by its vapor density or its freezing-point depression, and molecular formulas were determined by elemental analysis, a technique that determined the relative proportions of the elements present in the compound. These were long and tedious techniques that required relatively large amounts of a very pure sample. Today, molecular weights and molecular formulas can be rapidly determined by mass spectrometry from a very small amount of a sample. 

The coupled NMR spectrum (Fig. 6b) and the DEPT subspectra (Fig. 8c, d) for unequivocal detection of CFf multiplicities (C, CH, CH2, CH ) show that acanthifo-lin contains six non-protonated (Ce), four CH (C4//4), three CH2 (CsHe) and four CHs carbon atoms (04 12). These fragments ( Ce + C4H4 + C He + C Hn ) sum up to the CFf partial elementary composition CnH22 (Table 9, p. 164) in accordance with the molecular formula CnH220i, determined by mass spectrometry. In conclusion, no OFf group is present in the molecule. 

The exact molecular weight of pimaricin 73 (Scheme 73) (as the iV-acetyl derivative) was determined by mass spectrometry of its tri-methylsilyl derivative, which was sufficiently volatile and thermally stable. A revised structure for the antibiotic is based on such studies (107). The mass spectra of 0-trimethylsilyl pimaricin and several of its derivatives correspond to a molecular formula of C88H47N07 for pi-... 

I One of the first steps in determining molecular structure is establishing the molecular formula. In the past, this task was most commonly done by elemental analysis, combustion analysis to determine percent composition, molecular weight determination, and so forth. More commonly today, molecular weight and molecular formula are determined by mass spectrometry (Chapter 14). In the examples that follow, we assume that the molecular formula of any unknown compound has already been determined, and we proceed from that point using spectral analysis to determine a structural formula. 

Borris et al. (335) have reinvestigated the structure of melinonine E, whose isolation was originally reported by Bachli et al. (309), with the aid of more modern techniques (335). The molecular formula of the base, determined by mass spectrometry, has been revised from C2oH23N20 to Ci9H2iN20 . Detailed analysis of the and NMR spectra leads to structure 15a, that of an anhydronium base with a new skeletal arrangement. Doubling of the... 

To find the molecular formula of a compound, we need one more piece of information—its molar mass. Then all we have to do is to calculate how many empirical formula units are needed to account for the molar mass. One of the best ways of determining the molar mass of an organic compound is by mass spectrometry. We saw this technique applied to atoms in Section B. It can be applied to molecules, too and, although there are important changes of detail, the technique is essentially the same. 

In recent years the application of electrospray ionization (ESI) mass spectrometry, quadrupole time-of-flight (QqTOF) mass spectrometry, and Fourier transform ion cyclotron resonance (FT-ICR) are used for further structural characterization of DOM (Kujawinski et al., 2002 Kim et al., 2003 Stenson et al., 2003 Koch et al., 2005 Tremblay et al., 2007 Reemtsma et al., 2008). MS/MS capabilities provide the screening for selected ions, and FT-ICR allows exact molecular formula determination for selected peaks. In addition, SEC can be coupled to ESI and FTICR-MS to study different DOM fractions. Homologous series of structures can be revealed, and many pairs of peaks differ by the exact masses of -H2, -O, or -CH2. Several thousand molecular formulas in the mass range of up to more than 600 Da can be identified and reproduced in element ratio plots (O/C versus H/C plots). Limitations of ESI used by SEC-MS are shown by These and Reemtsma (2003). 

The Infrared Region 515 12-4 Molecular Vibrations 516 12-5 IR-Active and IR-lnactive Vibrations 518 12-6 Measurement of the IR Spectrum 519 12-7 Infrared Spectroscopy of Hydrocarbons 522 12-8 Characteristic Absorptions of Alcohols and Amines 527 12-9 Characteristic Absorptions of Carbonyl Compounds 528 12-10 Characteristic Absorptions of C—N Bonds 533 12-11 Simplified Summary of IR Stretching Frequencies 535 12-12 Reading and Interpreting IR Spectra (Solved Problems) 537 12-13 Introduction to Mass Spectrometry 541 12-14 Determination of the Molecular Formula by Mass Spectrometry 545... 

Rationalization of known compounds provides a level of usefulness that justifies the rule. But the rule also permits observed molecular stoichiometries of newly synthesized compounds to be translated into acluster shape. For example, [Al Bu ]2-has eve = 50 or sep =13 consistent with n = 12 and a deltahedral structure. The compound has been synthesized and an X-ray diffraction study reveals an icosahe-dral shape. The ability to suggest reasonable structures based on knowledge of a molecular formula generated by a technique like mass spectrometry accelerated the development of cluster chemistry simply because rapid spectroscopic methods can be more productively applied. Although efficient X-ray crystallographic structure determination reduces its importance for compounds that can be isolated in pure crystalline forms, transient intermediates detected in a reaction mixture can now be given reasonable structures. 

To find the molecular formula, we must determine the molecular weight today, almost certainly by mass spectrometry, which gives an exact value (Sec. 13.2). Ethane, for example, has an empirical formula of CH3. A molecular weight of 30 is found, indicating that, of the possible molecular formulas, C2H6 must be the correct one. 

Methods of characterization aim to determine the products of a reaction. The level of detail expected depends on the circumstances, and determines the range of methods required. If the aim has been to make a known compound, one needs to check its identity and purity. Fingerprinting techniques measure a spectrum or some other property and compare it with results published for known compounds and available in literature databases. Such techniques may also show whether impurities are present, but it is often desirable to check the purity of the compound independently, for example by elemental analysis. However, if the compound prepared is a new one, more thorough investigation is appropriate. The stoichiometric formula may be found by elemental analysis, and the full molecular formula in principle by mass spectrometry (MS). MS combined with other spectroscopic techniques, especially infrared (IR) and nuclear magnetic... 

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