Why Is It So? the [.Sup.1]H-NMR [CH.Sub.2] Splitting in Substituted Propanes

Article excerpt

Nuclear magnetic resonance (NMR) spectroscopy is an important tool in the structural analysis of both organic and inorganic molecules. Proton NMR spectra can yield information about the chemical or bonding environment surrounding various protons, the number of protons in those environments, and the number of neighbouring protons around each proton. However, there is a common misconception about the relationship between the splitting of signals due to the neighbouring protons and the (n+1) rule. This paper discusses how the appearance of deceptively simple spectra has led to this misconception and the correct interpretation and application of the (n+1) rule.

INTRODUCTION

Nuclear magnetic resonance (NMR) spectroscopy is an important tool in the structural analysis of both organic and inorganic molecules. It has been in the university chemistry curriculum for several decades and is now appearing in the high school curricula of more jurisdictions. For example, NMR spectroscopy is included in:

* the Victorian Certificate of Education (VCAA, 2005) as a compulsory topic for all students in Chemistry Unit 3. Analysis of splitting patterns is explicitly mentioned in the VCE Study Design;

* the International Baccalaureate chemistry option A (modern analytical chemistry) (IBO, 2007). In particular, splitting patterns are part of the option for the higher level (HL) stream; and

* the draft Australian National Curriculum (ACARA, 2010) as one of a list of analytical techniques in Chemistry Unit 4. Students have to study two techniques from the list.

Proton ([.sup.1]H) NMR spectra yield information about the chemical or bonding environment surrounding various protons, the number of protons in those environments, and the number of neighbouring protons around each proton.

The [.sup.1]H NMR spectrum of 1-chloropropane (Figure 1) is typical of the spectra of a small organic molecule (Figure 2). There are a number of different signals, each corresponding to protons in different bonding environments (non-equivalent protons). The interactions of neighbouring non-equivalent protons results in some or all of the signals being split into a number of peaks. The signal at 1.61 ppm appears to be split into a sextet (6 peaks in the signal], giving rise to the misconception:

that: the central [CH.sub.2] is surrounded by a total of 5 protons - two protons in the [CH.sub.2]Cl group and three protons in the [CH.sub.3]. This gives rise to a splitting into (5+1) peaks - a sextet.

This paper discusses the '(n+1) rule': the signal in a simple [.sup.1]H NMR spectrum is split into (n+1) peaks, where n is the number of protons on neighbouring atoms that are non-ewuivalent to the signal proton (Hammes, 2005; Kemp, 1986, 1991: Lambert, Shurvell, Lightner, & Cooks, 1998; Nelson, 2003) and its correct interpretation.

THE 1-CHLOROPROPANE SPECTRUM

The 1 chloropropane spectrum is often used as a textbook example because the carbons are numbered 1-3 from left to right as shown in Figure 2, and their associated NMR signals are also in the same order from left to right as shown in Figure 1.

[FIGURE 1 OMITTED]

Protons that are bonded to adjacent atoms (eg hydrogens in positions A and B in Figure 2) can interact with one another so that their signals are split into a number of closely spaced peaks. The exact theoretical basis of this is not required at Year 12, but interested readers can find the information in books on NMR spectroscopy (Kemp, 1986, 1991; Lambert, et al., 1998; Nelson, 2003). In simple spectra, the peaks are split according to the (n+1) rule, stated above. The confusion and misconception arises because the caveat 'in simple spectra' and the definition of 'simple spectra' are not given in Year 12 textbooks, or in most first year university texts.

[FIGURE 2 OMITTED]

A simple [.sup.1]H NMR spectrum is one in which protons are either not interacting with other protons, or interacting with (at most) one set of non equivalent protons. …