Atomic charges ambiguity

Distances - both, calculated and those from crystal structures - are ambiguous, since significant bonding to silicon can occur at large separations (e.g., SiR3 " toluene complex, 2.18 A) [3, 7]. Atomic charges depend on the method of calculation and are not easily interpreted. Silicon is much more cationic than carbon in similar structures. Electropositive silicon is always quite charged in neutral and even in the hypervalent anions and dianions (Fig. 1). 

C is indeed more electronegative than neutral O. Similar ambiguities arise in open-sheU neutral species (where atoms may have different effective spin-charges in the two spin sets), ylidic species (where formal charges alter the usual neutral atom properties), and other unusual atomic charge states. 

Mass spectrometry is a sensitive analytical technique which is able to quantify known analytes and to identify unknown molecules at the picomoles or femto-moles level. A fundamental requirement is that atoms or molecules are ionized and analyzed as gas phase ions which are characterized by their mass (m) and charge (z). A mass spectrometer is an instrument which measures precisely the abundance of molecules which have been converted to ions. In a mass spectrum m/z is used as the dimensionless quantity that is an independent variable. There is still some ambiguity how the x-axis of the mass spectrum should be defined. Mass to charge ratio should not lo longer be used because the quantity measured is not the quotient of the ion s mass to its electric charge. Also, the use of the Thomson unit (Th) is considered obsolete [15, 16]. Typically, a mass spectrometer is formed by the following components (i) a sample introduction device (direct probe inlet, liquid interface), (ii) a source to produce ions, (iii) one or several mass analyzers, (iv) a detector to measure the abundance of ions, (v) a computerized system for data treatment (Fig. 1.1). 

Charge numbers, oxidation numbers, and ionic proportions. When the oxidation number of the central atom can be defined without ambiguity, it may be indicated by appending a roman numeral to the central atom name.2 This number is enclosed in parentheses after the part of the name denoting the central atom. No positive sign is used. When necessary a negative sign is placed before the number. Arabic zero indicates the zero oxidation number. No space is left between this number and the rest of the name. 

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